AVG Anti-spyware free version is an excellent program to have as on on-demand scanner. From what I've read, Spyware Terminator - there is a rogue version that is very bad by the way so be sure that you DO NOT get that one - is quite good at real-time protection, but not so hot as an on-demand scanner. One of our members is currently testing this program out.
I'd suggest looking through this topic
for several free applications.
Basic rules of thumb:
Use 1 Anti-virus as more than one creates many problems and may make your computer more vulnerable.
Use several on-demand anti-spyware programs as one will catch what others won't.
Use 1 firewall as more than one will make your computer less secure and may cause other problems.