Why is my computer so slow now?

Ok I have an Hp pavilion a000 with 512 mb of ram and a 120gb hard drive. And somehow the os got wiped off and we got windows xp sp 3 put on it. Well it worked fast at first and now it takes forever and is glitchy. I do a weekly virus scan, I've tried defraging the hard drive. Disk clean up, programs like speeditupfree and nothing will help my computers speed. What can I do to speed it up please help!!!! The main thing is I want to play farmville on it and it just lags and glitches the whole time. Still playable but glitchy and slow