"GERRY": Usually a complete reinstall of your system. Not just installing a new windows. But I'd use a registry cleaner and window washer first, then if that doesn't improve, try a program called combofix which will do a deep scan of your system, but make sure you back up your registry as per the programs instructions first. It'll eventually give you a text document showing everything it's done and what possible probmatic items maybe sitting in your registry. Then you can use reg edit to fix anything that looks dodgy.

If that fails... you'll need to format your hard disk and do a complete re-install.