Norton springs to mind as being a possibility - it's a bit of a resource hog. That said, it shouldn't slow you down that noticeably. The truth is it could be any number of things - you could spend months getting to the root cause of the issue.
...or you could just back everything up, wipe your disk and then start again.
I'd honestly pick the latter approach. It doesn't take long and could save you a lot of headache in the long run. Plus, you get a nice squeaky clean windows install without any rubbish clogging it up