Hmm... Strange problem. The fact that it works in safe mode suggests it could be some kind of virus or something. Do you have protection programs?
Try pressing F8 at bootup to come up with the safe mode menu, and select 'Last knwon configurations that worked' and try that.
If that doesn't, is it a problem that has suddenly started happening? If so, go to System Restore. It should mention this when it starts running in safe mode. Or go to start, Accessories, System Tools, System Restore. Take the computer back if you've allowed it to to one a few days ago or where you think the problem started happening. After it restarts, hopefully things will work fine.
Don't worry, system restore only changes settings you might have changed. It doesn't delete any files, though if you've installe programs in that time, they'll delete from the registry so they can't be ran the normal way. But other than that, you might get a working system back
Hope this helps