best NON-INTRUSIVE antivirus program?

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

It is the nature of an AV program to be intrusive altough you can customize AVG, which is free, to work only on command. Then again I can't promise resources won't dip a little. XP is not so reliant on system resources as 98. You may instead want to consider adding more RAM or cancelling out some services that don't need to be running.

housecall.antivirus.com

My favorite.