Thought I would add my two penneth worth herel
try AVG from www.grisoft.com
They offer a FREE version (inluding unlimited updates) for the home user.
Very low overhead and works very well.
Also, the MS Anti-Spyware bet is still free at the moment.
For an all-in-one solution www.zonelabs.com ZoneAlarm takes some beating.
All the best
Cameron