ccleaner is a great tool, download it and run it to remove a load of un-needed crap off of your machine.

Then delete stuff you don't use any more. Click the start menu then in the search box type in uninstall. Click the 'uninstall a program'. Run through the list and delete anything you have installed but don't use.

Restart your computer to complete uninstallation of some programs that may need it, then in the same search box, type in defragment. Open up the disk defragmenter and defragment your C drive. Depending on how bad it is it could take a couple of hours, so just leave it to run and don't be tempted to cancel it half way through.

After that, you should find your machine running a bit better!