1. If you have a Windows computer, you must obtain and install all of the following: a reputable antivirus program, a software firewall, a junk-mail filter and an antispyware program. Even if you own a Macintosh (Macs have been unaffected by most of these threats to date), you will still need to turn on your computer’s firewall and employ a junk-mail filter.

2. Upgrade to the latest versions of the leading Windows web browsers, Microsoft’s Internet Explorer 7 and Mozilla’s Firefox 2.0, both of which warn you when a web page you’re visiting appears to be phony. (The new Internet Explorer also has under-the-hood security improvements that close some of the holes plaguing older versions.) You might also consider add-on software, like McAfee’s SiteAdvisor or the new Norton Confidential, which warn about fraudulent sites and, in the case of Norton, also about malicious software on your PC. On a Mac, consider using Firefox 2.0 instead of Apple’s Safari, which, while very good and generally secure, lacks a fake-web-site detector.

3. Never respond to or click a link within any unsolicited email message from a financial institution — even your own — no matter how official it looks. Crooks have become skilled at mimicking logos and typefaces used by banks, brokers and payment services like PayPal. When you click on links within these fake emails, you’ll be taken to web pages that look like the companies’ official sites, even down to the address, but they’ll steal your log-in information. Be especially wary of email from a financial institution that asks for account information or says you must log in at a linked site to address a problem. You can phone the company to see if there really is an issue. Obviously, this caution doesn’t apply to some financial emails, such as confirmations of online stock trades you’ve just executed. But in general, you shouldn’t conduct financial transactions via email or links in email. Instead, go directly to the financial sites you use.

4. Similarly, never act on emails offering stock tips, miracle pills or the chance to earn money by storing millions from overseas in your bank account. Sounds obvious, but in the past these scams might have cost you a little money. Now they may be part of more-damaging identity-theft schemes. Treat such come-ons the way you’d treat a stranger in a bad neighborhood who made such promises.

5. Never, ever download software from a company or web site whose honesty or veracity you’re not sure of. If a site says you’ll need special software to use its features, don’t bite. Even if the software is well known and safe — like RealNetworks’ RealPlayer, Apple’s QuickTime or Adobe Flash — don’t get it from a link provided by a random web page. Instead, visit the Real, Apple or Adobe sites to download it manually.

6. Finally, never use security software offered to you via unsolicited email or a popup window, or that suddenly appears on your PC. Such programs are almost always scams and often install malicious spyware, adware and viruses rather than cleaning them up. In general, stick with leading security brands like Symantec, McAfee, Zone Labs and Webroot. Check the software in the reviews section of PC Magazine or the CNET web site. If it isn’t covered there, it’s probably untrustworthy.

These creeps now find it easier than ever to defraud people, steal their identities and blast them with unwanted or false advertising. They use the Web as a pathway to infect computers, corrupt data and take over others’ machines.

Security software can help block this wave of woe. But it would be better to know in advance if a Web site that comes up in a search result, or one you arrived at through other means, is harboring malicious software, or perpetrating scams, or generating spam and unwanted pop-ups. It might also be nice to know if a site with an innocuous name contains pornography, hate speech or other content that might be offensive to you.

I’ve been testing two services that aim to provide such advance notice of bad or offensive sites. The services, Scandoo and SiteAdvisor, take different approaches to the task and offer different features. But both instantly mark up a search-result page, and label the links that might be dangerous.

Both services are free of charge, and each works on both Windows and Macintosh computers, and in multiple Web browsers. On balance, I prefer SiteAdvisor, though Scandoo has a couple of things SiteAdvisor lacks.

Scandoo, still in beta, or test, phase, is from a company called ScanSafe, which provides site-scanning and security services for corporations. SiteAdvisor was founded by some engineers from MIT and was recently bought by McAfee, the big computer-security firm.

SiteAdvisor works via a software plug-in that you download and install. The plug-in, available at www.siteadvisor.com, modifies either the Internet Explorer browser for Windows, or the Firefox browser for Windows, Macintosh and Linux, so the browser can identify bad Web sites. SiteAdvisor works with the Google, Yahoo and MSN search engines.

Scandoo requires no software downloads and works with more browsers than SiteAdvisor does. But it requires you to enter a search term at its Web page, www.scandoo.com, rather than at the home page or search box of your favorite search engine. It then transfers to the search engine you choose and modifies the results page to identify sites that may be troublesome. It now works only with Google or MSN.

There are some other major differences between the two. Scandoo scans Web pages on the fly to look for bad stuff. SiteAdvisor matches Web sites against a database it has compiled about content. Scandoo works only on pure search results, not the ads alongside the results. SiteAdvisor rates the results and the ads, which often are more dangerous.

In addition, because it is built into the browser, SiteAdvisor can rate any site you are visiting, not just sites listed in search results. SiteAdvisor places a small, unobtrusive icon in your browser. The icon is green if you are on a Web page it considers safe and honest. It turns red if it regards the site as dangerous.

Scandoo works only on search results pages. But it has a function SiteAdvisor lacks. It can rate pages for offensive content, while SiteAdvisor focuses just on the presence of malicious software, or invasive advertising techniques. Scandoo allows you to specify which kinds of content you want flagged, including pornography, hate speech and gambling.

SiteAdvisor also flags sites it regards as perpetrating scams, like charging people for software that actually is free. But in my tests, it ignored some other scams, such as offers for pills that magically enlarge body parts.

In my tests, SiteAdvisor consistently flagged more Web sites as bad than Scandoo did. When I searched for “Free iPods” in Google, Scandoo gave all the regular search results a green check mark, meaning OK. SiteAdvisor marked the first regular result in red and gave it an “X,” meaning trouble. It also marked most of the ads in red and gave them “X’s.”

This is partly due to different techniques they use. Scandoo claims its real-time scanning can uncover bad sites SiteAdvisor might miss. SiteAdvisor claims its database is more comprehensive.

Another reason for the disparity is that SiteAdvisor isn’t just looking for viruses or spyware. It uses test computers to see if sites are likely to generate what it calls “spammy” email or pop-up ads. If they do, the sites get flagged.

Some might regard SiteAdvisor’s filters as too aggressive, but, unlike Scandoo, it gives a detailed explanation for each rating. The explanations I saw made sense. For the free iPods site SiteAdvisor flagged, it explained: “After entering our e-mail address on this site, we received 11 e-mails per week. They were very spammy.” It even showed some test emails.

Both services are very helpful. You might want to use Scandoo if you’re concerned about offensive content. But for flagging malicious software and invasive advertising, SiteAdvisor is more comprehensive and tougher.

• Email me at mossberg@wsj.com.