Jonathan said:
An epic amount of things could be causing this, your best bet is taking it in to get repaired
NOOOOOOOO. No. NO. NO!!!.
Never trust a computer repair person. Last one I had said the computer would run slow, when it shouldn't after removing one. Obviously if you reinstall windows, it should run like new. It didn't, after smashing it to peices the other day because I was throwing it out and didn't want data stolen (and it was partly fun) I was taking the ram out as it is useful, and noticed one of the RAM chips was missing. The right side, he had obviously taken it resulting in my computer being slow.
When in doubt, reinstall windows. Keep back ups of all your files, and have a auto running anti virus on a disc somewhere.