You say you don't have a lot of processes running . . . but nearly every time I look at a PC that's running slow, there's a ton of crap on it that runs at different times. Toolbars, p2p, games checking for updates and locking up the cpu until the connection is made, all kinds of crap, old virus scanners with expired subscriptions, a pair of dirty sox . . .
Look there first, uninstall or cripple everything that wasn't there when it was new, and see what happens. By all means clean up unused files, Defrag, and check how close you are to filling up your C drive.
It costs only your time and may save you the cost of a new psu that may not fix your problem.