Try tapping the F8 key during the initial bootup and see if you can enter Safe Mode. If yes, disconnect all external devices and try rebooting normally. If this works, then reconnect each device one at a time until you find the problem.  This is a simple and easy first step to try. If none of that works then there are other problems.

did you see your mouse pointer when you're on the black screen? If you're able to go to windows. You can try using system restore by answering No on the yes or no questions that will appear at the beginning of the safe mode and restore it to an earlier date.

or

go to start->all programs->accesories->system tools->system restore