For 64-bit Windows 7 I use Avast Home Edition with SpyBot-S&D, MalwareByte's Anti-Malware and PrevX Free.
Avast is highly configurable and none of this has any impact on my system what so ever. But it does provide a good combination to cover all the bases.
I also use OpenDNS.com with a free service account for my DNS service which adds an extra layer of protection.
I use this list for OpenDNS settings. Block list 25
For web browsing I use FireFox with the Adblock Plus