I use SpyBot and Adaware regularly.
I also use AVG free anti-virus program from grisoft.com
It is less draining and obtrusive on my system than Norton was and has kept me virus free for over 2 years.
Of course, as someone mentioned earlier you've
got to make sure whatever anti-virus program you are using is updated regularly. Not updating is little better than not having an anti-virus program at all.
Here's a good page for tools and info:
http://geekytips.com/virus.html