Honest answer, don't know?
Could try this, mate I have says it's great, but not tried it myself yet:
http://www.bugdoctor.com/
Or it could be virus checkers running in background, or combination of programmes.
Spec of my PC not much higher than yours, and itself was running slow yesterday, complete re-install last night, hey presto, full speed ahead once more, maybe time to spring clean yours?