There are so many things that could be effecting your performance on your user-name that it'd be nearly impossible to assist you in pinpointing it without extreme effort(which I'm not really inclined to exert).
However, one quick and easy thing you could do is create a new user on the computer, and only use that username to play your games. Don't serf the web on it, don't download crap on it, don't install other programs, etc.
This should keep it a clean username, and your performance shouldn't be effected by all the junk on your other username. Make sure you go back to the old username if you want to surf the web, download, or w/e.
