So, yeah, there's actually quite a few antivirus apps for Windows XP that are up-to-date.
I find that AVG Antivirus is my favorite.
There's also the open-source ClamTk built on ClamAV. (which, by the way, is VERY low power)
There's also one called Webroot that doesn't seem to have a free version...
But, yeah, if you really want to have a lightweight AV that won't eat up your resources, maybe consider the Windows version of ClamTK. If you do go with it, bear in mind that it doesn't report to the Security Center, so Security Center might think you don't have antivirus.