Programes are not opening on PC, like iTunes, Task Manager, Volume mixer?

I am an amateur DJ, the last thing I what to do is factory restore, I would lose up to 300 songs! Loads of programes aren't opening, the last thing I intalled was audacity, which was a bit dodgy. Can someone tell me what iv got to do to get this back to normal. System restore also won't open, I'm running vista :(