[pecenipicek]

Windows built-in Firewall + MS security Essentials.

This. especially considering the fact that the win firewall got a lot better in 7. not sure how is it in vista tho

[Ravenholme]

Spybot Search & Destroy is pretty neat, too. No program ever will harm your registry, if you don't tell spy bot that it's allowed to do so

My security suite is Windows Firewall, MSE, AVAST!, Malware-bytes anti-malware and Spybot S&D. Works pretty damn well, I've only had one scare recently and Avast wrapped it up nicely after some measured firefighting.