If you've been infected with any of the most recent Windows viruses, you can download a free mini-scanner/cleaner from Network Associates. This free application, called Stinger, isn't a substitute for a virus scanner. It's the functional equivalent of having to go to the emergency room rather then having regular checkups. If you don't know if you're infected – download it and double check.
Stinger will not detect all virus threats, just the most popular. For better protection, get a free virus scanner and –by all means, people– run it occassionally. Also, you have to let scanners update the definition files – once a week is probably a good schedule. Otherwise your just scanning for old stuff and missing anything new!
Here's a list of virii that Stinger currently detects and cleans:
Stinger found a virus that it couldn't repair. Why is this?
This is most likely due to Windows System Restore functionality having a lock on the infected file. WindowsME/XP users should disable system restore prior to scanning.