Let's try a repair install of iTunes.
Restart the PC first.
If you're using Vista or 7, now head into your Uninstall a program control panel, select "iTunes" and then click "Repair".
If you're using XP, head into your Add or Remove Programs control panel, select "iTunes", and click "Change". Select "Repair" and click "Next" as per the following screenshot:
Can you launch your iTunes now?