I think u should fully optimize ur PC...as may be registries also responsible..which slow down ur PC as they takes a big space in background...there is this one application named Advanced System Optimizer..which can definitely help u in optimizing PC..as has loads of features.. like..
i m also using it for regular maintenance for my laptop.. it a best tuneup tool i used so far..! i haven't face any prob..after using this application..!!
It increases my PC performance effectively..booting time reduces from 5 to 2to3 min.!!
can download it 4m Cnet.
I personally like to stay away from the registry optimizers myself. Only time you should be doing anything in the registry is if you are having actual problems that are related to the registry. but if your problem is the computer is running slow the registry does not need to be touched. A good place to start is good ol hijackthis. It can be used for more then just virus infections. If you can see if you can post a hijackthis log here. Can take a look at what startups and services you have running that don't need to be running, along with any bho's and toolbars we can clean out.