I have Windows XP installed on an older system. Any suggestions for a free, lightweight anti virus program?
Antivirus programs tend to be worse on a computer than most viruses.
I've found that the the free version of AVG Anti-Virus is very lightweight. I use it on virtual machines a lot, and it doesn't slow them down much at all.
Don't download any viruses or use any applications that will download them for you :)
I admit that is a slightly facetious response, but by avoiding using IE and Outlook, keeping everything patched, having a decent firewall between me and the internet and not downloading very much to my computer (usually sticking to Open Source projects and paying close attention to checksums on everything I download ) I have managed to get by for the last eight years or so without needing more than the occasional AV or Spybot S&D scan, both of which have yet to show any problems.