Have you reformatted and reinstalled Windows recently? I'm a big believer in doing this every 6-8 months if only as an excuse to clean out what software is installed on my machine.
On that note, if you're uninstalling apps, use Wise Program Uninstaller or IOBit Uninstaller. Both of these will do a search for all the leftover registry keys and such that can clutter your machine.
Also take a look at your startup items if you haven't recently. Having a lot of items running in the background will definitely slow your computer. Try to keep your startup items to a minimum (so turn of Spotify, Last.fm, etc unless you use them every time you're on your PC). You can actually access the list of startup items from CCleaner (link in dave_l's post.
I'll think some more no this when I haven't just woken up and try to get you some more tips.