|Windows 7 or Vista?|
Did you add or remove any software prior to the problem?
Did the problem just start suddenly or did you have other odd things happening beforehand?
Most likely suspect is malware, followed by a hardware problem.
From the safe mode menu, is the option to repair your computer available. If it is, select it. It may try to automatically fix problem - if it does, let it and then see if any better when its finished. If its no better, go back to repair feature and cancel out of automatic repair if it tries again. This should bring up a screen with several options - including system restore. Select this and choose a restore point from before you had the problem. It will restore & restart (though if it was malware, it may not show any restore points, and even if it does & you choose one, it may say its failed after completion) - but hopefully it will boot as normal after restart. if safe mode menu doesn't include reapir option, you will need vista or win7 install DVD (to match what you actually have) to boot from and start repair from there instead. Vista & win 7 both have basically similar features here.