I'm using 'Webroot SecureAnywhere'.
http://www.webroot.com/En_US/sites/l...iday-2012.html

The install was small enough to fit on a floppy.
It only uses 400K of system Ram and 0.01% of my CPU.

Unlike a traditional Anti-Virus, it doesn't have a definition file which it compares files with.
Rather it follows heuristic principles, basically checking if files follow behavior typical of a virus... such as a file trying to record your keystrokes, save your personal data, wanting to modify the registry, wanting to be started on boot-up, etc.
An average full system scan, takes my system 27 seconds.