Do yourself a favor and just back up what you have and reinstall windows. You will have a PC running like new and pretty much get rid of ALL problems (Unless it's hardware related).
I have a custom rig and every couple years I just reinstall windows. I add new parts to this PC, so I Have to reinstall from time to time in order to keep it running like new. Most people just buy new PCs when theirs starts acting up, but when you have $2800 invested in building one, that's out of the question.
*Edit* On a last note, keep in mind that W7 will no longer be supported by MS after this year, so if you have to buy a windows disc, don't buy 7, even though W8 blows big balls and W7 blows it away in performance and ease of use. Not sure if there is a W9 coming soon