There are quite a few known sites that offer "free scans" or "anti-spyware" software that actually load crud on you. These usually show up as popups while you're surfing. If you don't know them, don't go there.

The known and reputable AV software sites nearly all do offer "web scans" and are almost indispensible if you've got a problem, since the "new varieties" of worms often attempt to disable the AV you have installed on your own machinge.

Rapaire's comment should be noted, however. The report is that nearly all of the websites that offer a free AV check download a "bot" to your machine that actually runs the check. The bot, a small program, stays on your machine, and nearly all of the bots are susceptible, if you visit a malicious site, to a buffer overflow that could allow someone to take over your machine. Most of the reputable AV "free check" sites claim to have "fixes" that will be installed if you return to the site and run a new "free virus check," or have downloadable "repairs;" but remembering which sites you may have used may be a problem for many of us, and you'll need to confirm that the one you used has fixed the problem - for each and every one you've used - if you really want to feel safe.

John