For a limited time you can download and use AVG 7.5 Professional Antivirus for free. For those of you running the always-free version of AVG there are some advantages to making the upgrade, such as advanced scanning and scheduling options.
AVG 7.5 Professional retails for $29.95, and this offer is good until January 17, 2008.
Homepage: AVG Home
The file that you download will automatically enter in a valid serial number during the installation process.
I realize that a lot of people don't use AVG, but it is a pretty good program overall and I have it running on the other XP in my dual-boot machine. The license is valid until 12/01/2008 (at least in my case).