Why is my computer so slow?

Alright, my computer is a good one, an optiplex 320, not the greatest but its good to play video games since i upgraded the video card to a new nvidia one and i have 2gb of ram, it was working fine to play burnout paradise, cod4, world of warcraft, and tons others, but just recently my computer started to get really slow! loading youtube vids is slow, even when i open cod4 and im not even connected to the internet yet its really slow! i ran norton 360, spyware doctor, malwarebytes, and a disk defrag, and my comp is still REALLY slow! Please help because i really wanna go back to playin games on this computer seeing as i payed dam near $400 for new specs, ill be checking back every 5-10mins to choose the best answers, thanks for reading my essay :P