Depending on how much of a clusterforget your harddrive is, you may need to restore.
Just do a disc cleanup, then a defrag and then another disc cleanup. Keeps my PC ship-shape and running fast-ish.
Delete any of the stuff that comes prepackaged with your computer. That includes most manufacturer-made software like what HP throws in.
Backup any documents or projects or research or whatever onto flash drives or CDs or something. You don't really need those sitting on your computer 24/7.