I'm using AVG Antivirus and the 1 process is using 5k of memory usage, compared to the 38k of memory usage Norton AV is using.
I don't bother with any others because I'm happy with AVG, so I reccomend it.
http://free.grisoft.com/doc/2/lng/us/tpl/v5
http://www.download.com/AVG-Anti-Virus-Free-Editi...39_4-10320142.html