I am a system administrator so I see all types of computer problems
I usually use a combo of symantec anti virus and a program call avant gaurd.
Avant guard used to becalled ewido and its free and it works rather well.
http://www.grisoft.com/ww.download-trial even though its says 30 day trial that is just for auto scanning after the 30 days you just have to manually do a scan if you feel there is problems.
also I always do a scan in safe mode first then when windows is fully running cause sometimes in safemode it will delete certain virus or trojans that have problems getting removed while all the services are running.