That's hard to answer without more details. First, how old is your PC, and have you made any modifications to it recently; and second, have you downloaded anything recently (e-mail attachments, pictures, programs) which might have contained some form of malware? Since it's booting into safe mode, it's possible that you have a virus or a Trojan which is running itself as part of your PC's boot-up process. Try running a virus scan from safe mode. If that doesn't fix your problem, you might have to try booting into Linux or wiping your drive. Given that safe mode works, there are few (if any) hardware problems I can think of that might be responsible.