I would recommend downloading Microsoft Security Essentials and removing AVG. It is actually very good and rates up there with the top paid AV's.
It is free, as long as you have a legit Windows. Just google MSE.
Also, if you haven't tried this macro, try lowering your spell effect level via this macro /console SpellEffectLevel 25
Note: play with the number
Crank it up and it looks pretty cool, but will bog down any system.
Also, what is the temperature of your video card
while raiding? You can check using the Catalyst Control Center that comes with the ATi driver package.
One more thing, do you have Indexing enabled on your HDDs? (open My Computer, right click on any HDD, Properties... it's the tick box at the bottom...make sure it's unchecked)
Good luck!