The result is that criminals can use the browser as a gateway to plant all kinds of bad software on your PC and to steal your personal information by accessing your files or recording what you type.

Today’s common defenses are hit or miss. You can install antivirus and antispyware software, but those must be updated constantly to recognize new attack patterns, and sometimes the bad guys get ahead of the defenders.

Now, a small Silicon Valley company called GreenBorder is trying a radical approach. It has developed a product that isolates the browser from the rest of the computer, without impairing your Web browsing. It erects a sort of fence around the browser. Inside that fence, the browser runs normally, along with associated programs like media players. But the browser can’t be used to install bad software or to spy on the rest of the computer outside the fence.

With this product, called GreenBorder Pro, any malicious software you pick up is trapped in a computing environment — called a virtual session — that exists only inside the fence and can’t affect any key files or settings outside.

Once you quit the browser, this virtual session simply disappears, along with any bad stuff that has collected within it. Your files and settings remain unaffected. You can even purge the bad stuff from the virtual environment at any time by clicking on a command called Clean and Reset GreenBorder.

For instance, inside a GreenBorder Pro session, you might get infected with a “browser hijacker,” a spyware program that permanently changes your browser’s home page and search page to sites operated by sleazy companies. But once you quit the browser, or click on Clean and Reset, the hijacking effects disappear.

To show it’s protecting you, GreenBorder Pro places a literal green border around the browser. It also displays a summary of the malicious software it trapped inside its border and prevented from affecting your files and settings. If you deliberately choose to download a file, it gets placed in a special folder on your PC and marked with its own green border. It then runs in a protected session, just like the browser.

One big advantage of the GreenBorder Pro approach is that there’s no need to try to stay ahead of the bad guys’ techniques. GreenBorder doesn’t have to collect and update signatures of known bad programs.

GreenBorder Pro, available for download at greenborder.com, works only with Microsoft’s Internet Explorer browser for Windows, the most popular but least secure major Web browser. GreenBorder costs $50 a year, but is free for one year to the first 10,000 to download it.

I tested GreenBorder Pro on three different Windows computers, and found it easy to use. While it is difficult to test a negative, I deliberately used GreenBorder Pro to access numerous known bad Web sites, those that typically install unwanted adware and spyware on a PC.

Before starting, I ran both an antivirus and an antispyware scan to ensure each test machine was clean. I ran the scans again after quitting GreenBorder Pro, and the PCs were still clean, despite all those visits to bad sites. GreenBorder Pro reported that it had blocked dozens of attempted “file changes” and hundreds of attempted changes to the Windows Registry, the part of Windows that stores instructions for running software.

GreenBorder Pro has two other major features besides its basic function. One, called Privacy Zone, sets up a special browser mode for those times when you are visiting a Web site where you expect to be entering passwords or credit-card numbers. In this mode, the Clean and Reset process is performed automatically first, to purge any spy software that may be operating, and all traces of your Web activity are wiped out when you’re done. A yellow border is placed around the browser when you are in the Privacy Zone.

The second function is called SafeFiles. It lets you erect a fence around files from sources other than your browser, including email attachments and files you copied onto your PC. This is an extra-cost option priced at $14.95 a year.

There are some limitations to GreenBorder Pro. It won’t clean out existing viruses and spyware on your computer, or automatically block invasions that aren’t from Internet Explorer. You still need standard security software. It also won’t prevent you from being tricked into giving up private data. Also, some legitimate software downloads and installations will fail with GreenBorder Pro. You’ll have to run a browser in normal mode to get the downloads.

Also, I don’t see why you should pay $50 every year, instead of just once, for software that boasts that it doesn’t require updates. (The company says the annual payment covers better support and new features.)

But GreenBorder Pro is a novel and very promising weapon against malicious software.

• Email me at mossberg@wsj.com.

Now, there’s an even better approach to running Windows on a Mac. It’s called Parallels Desktop for Mac, and it’s from a small Herndon, Va., company called Parallels. It emerges from testing today and goes on sale for $79 at the company’s Web site, parallels.com.

I’ve been testing Parallels Desktop on a new MacBook Pro laptop, and have found it works very well, despite a few drawbacks. I prefer it to Apple’s solution, even though the Apple approach is free and also works very well.

Apple’s system, called Boot Camp, has one big limitation: It allows you to run only one of the two operating systems at a time, requiring you to reboot the computer to switch between them. As a result, you can’t quickly jump between Mac programs and Windows programs. You can’t, for instance, simultaneously download your corporate email in Outlook using Windows while editing a home video in iMovie using the Mac OS.

With Parallels Desktop for Mac, you can do this. You can run any combination of Mac and Windows programs at the same time, on the same screen. No rebooting is necessary. You can even cut and paste material between Mac and Windows programs, and share files between the two environments.

The Parallels approach, called virtualization, runs Windows, with all its features, inside a window in the Mac operating system. It creates a faux Windows PC, called a “virtual machine,” that co-exists with Mac OS X. You can devote the full screen to either operating system or you can reduce Windows, and whatever programs it’s running, to a window on the Mac that can be dragged anywhere on the screen and made as small or as large as you like.

Unlike Boot Camp, Parallels Desktop can run every version of Windows back to Windows 3.1, not just Windows XP. It can also run Linux and even older operating systems like OS/2 and MS-DOS. You can even create and run multiple virtual machines, with different operating systems inside, up to the limit of your Mac’s memory.

Virtualization isn’t a new concept, and it’s not even new on the Mac. Microsoft offers a product called Virtual PC for Mac that runs Windows inside a window on older, pre-Intel Macs. But Virtual PC runs painfully slowly on these older Macs, and it can’t run every Windows program. It doesn’t run at all on the new Intel-based Macs.

Parallels Desktop runs Windows a little more slowly than Apple’s Boot Camp does because it is accessing the Mac’s hardware through the Mac operating system rather than directly, as in a dual-boot system. But, in my tests, it was very snappy, as fast as many regular Windows computers.

Inside my virtual Windows machine, I was able to run programs like the Windows version of Microsoft Office, the Windows versions of the Firefox Web browser, iTunes, Adobe Reader, Google Earth and more. All worked well, as did Microsoft’s Internet Explorer Web browser, Google’s Picasa photo program and Google’s Google Talk instant-messaging software.

I was able to do email in Apple’s Mail program while simultaneously watching a baseball game in Internet Explorer inside my Parallels Desktop Windows virtual machine. I wrote part of this column in the Windows version of Microsoft Word and part in the Mac version, cutting and pasting between the two.

And, unlike Boot Camp, Parallels Desktop doesn’t require you to dedicate a fixed section, or “partition,” of your hard disk to Windows. Its virtual Windows computer is contained in a big Mac data file that uses only as much space as Windows needs.

Setting up Parallels Desktop is fast and easy, though some novice users may find the program’s terminology a little daunting. As with Boot Camp, you have to supply your own copy of Windows to install once you create the virtual machine. And because the virtual Windows machine created by Parallels behaves like a real Windows computer, you have to install the usual raft of Windows security software.

So what are the drawbacks? I couldn’t eject a CD in Parallels without switching back to the Mac environment. Parallels treats fast USB 2.0 ports on the Mac as if they were slower USB 1.1 ports. It doesn’t support Microsoft’s DirectX technology, which many games rely on. The company pledges to fix these things in future versions.

Also, if you elect to use the optional file-sharing feature, your Mac data files could be corrupted or erased by a Windows virus. For that reason, this feature is turned off by default.

Still, Parallels Desktop is a very good product and a pleasure to use. It’s like having two computers in one, the best of both worlds.

• Email me at mossberg@wsj.com.