I had a stage when my CA was laggy, and it seemed to be from a program called csrss.exe, and it was taking up lots of my CPU only when CA was running, but only for a while.. and then it just stopped :O
Try Ctrl+Alt+Delete, then click 'Processes' and end the ones that you don't need and are taking up CPU, like Firefox or media player.. you may already know this but I didn't a while ago