VHG has provided a great answer. The most popular combination of security software, with the 'regulars' here in AB's Technology section, is almost certainly AVG Free together with Zone Alarm Free. They cover all of the main security risks.
Despite its name, spyware isn't really a security risk. It's just a pain in the @rse because it tends to slow your PC down. VHG's suggestion of Adaware Personal is a sound one.
I've never found a need for something like WinPatrol but it won't do any harm to use it.
My only slight disagreement with VHG is over the need to run AVG periodically. AVG's default settings automatically scan e-mails and other downloads which might contain viruses so, theoretically, there's no need for periodic scans. (All the same, it's probably still a good idea).
Chris