During the boot sequence (i.e. before the Windows screen loads when you start your computer) press F8. That will take you into Safe Mode. Your monitor display will look very odd (because your video driver won't have been loaded) but you'll still be able to follow this path:
Start > Programs > Accessories > System Tools > System Restore.
When you get into System Restore, use the option to reset your computer's registry to a date before the problem arose. (Choose the last date available before things went wrong). That wil hopefully fix the problem for you.
System Restore doesn't remove files such as documents, photos or emails. They'll still be intact. But if you've installed any new software, or performed any software updates, between the restore point and the present, you'll need to repeat what you did.
Chris