What I am looking for is a small antivirus program that runs on my command ONLY, and the rest of the time takes 0% resources (no taskbar icons, no background processes, NOTHING!).

I should be able to get this for free, but there are so many options it is hard to choose one. The smaller and simpler the better, as long as it has good virus updates.

Recommendations are appreciated

P.S. I use windows XP