Have you checked for obvious things such as a defrag of the drive along with a disk clean up? Over time of installing and removing programs plenty of broken links and incomplete registry entries get left behind, all these will help to slow down your PC.
Give CCleaner a try if you haven't already as it is an excellent application for cleaning up and removing rubbish from the registry.
When you run it for the first time it will find a great deal to remove, let it do so but check in the options first to move any cookies that you want to keep over to the right-hand window or you will lose all your passwords for forums etc.
There are three tabs to click on and run, when it comes to the last tab, "Issues" run that a couple of times because in fixing the issues already there it will create some new ones, just run it again to clean these up. It is quite safe to tick all boxes and let CCleaner do its job. click here