Virus scanners ar definitely a good thing to be running, as are
firewalls.  I also use these two programs to detect and remove adware
and malware:
 
Ad-Aware        http://www.lavasoftusa.com/
 
Spybot          http://security.kolla.de/
 
I run both a few times per week to keep my system problem-free.  Both
programs are freeware.
 
Craig
[Posted in FML issue 4197]