Done anything to perhaps produce the situation - as in install anything recently?
Or have per chance run any cleaner-utils (ccleaner comes to mind; likewise any number of registry tools...)?
If you have a rough idea of when it started to happen... can you go back a restore point or two and see if it persists?
Also (a very long shot) when you boot up: try safe-mode and also last known good config? Who knows - either of those may also resolve it?
If you create a new user-account with "full Admin" permissions... does the problem persist with that new Admin account?