Computer Performing Very Slowly....?

This morning I woke up and tried to play some of my games. I noticed there was an enormous amount of lag. Not just slow, I couldn't see anything in the game. I have uninstalled, reinstalled the game and that made no difference. The computer's internet was running about about 1/3 the normal speed and I checked everything to do with the connection and it was working just fine. I ran 7 different virus scans and they found nothing. I deleted all the obsolete files and much more. I have no idea how to fix this problem. If you have encountered this problem and found a way to fix it, please tell me what to do.