If you feel like you've got nothing to lose, then try running CCleaner.
Free version should do plenty for you:
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That's gonna clean out all the places on your computer where stuff tends to accumulate. In a lot of ways, it's going to speed up your computer. It's also going to nuke a lot of old cache data.
It has a function to clean out your registry as well, but I would definitely recommend making a back up of your registry before you do that in case something gets broken.