Free antivirus software with free updates from Grisoft.

Some have said it has missed a couple viruses, but I'll take it over Norton or McAfee ANY day.

*edit*
For me, it's run just beautifully for about a year now. It will catch viruses very quickly (I downloaded a file on a P2P program and the instant I opened the folder it was in (about 5 seconds after it downloaded) the program found it. Plus unlike "Norton the CPU Hog", my machine doesn't appear to take a hit, even though it appears to monitor all the same stuff (email, downloads, etc.).