paid - KAV/KIS
free - AVG Free.
i too have the same config. except m/b (915g), i'm usin Kaspersky Internet Security , & sometimes ZAP. (ZAP is not in start-up)
KIS misses many adwares/spywares, but ZAP catches them

, happens only when you visit "
THOSE" sites often
do you have Zone Alarm Pro (ZAP) or its security suite (ZASS 7)?
if later, then stay with it, coz its the best package out there.
try clearing junk files, cleaning resigtry, defragmenting...
use TuneUp Utilities 2007 (paid) for registry cleaning & CCleaner for clearing junk files.