Can you boot into safe mode? If that does ok, the first place to start looking is at installed drivers. Have you updated any device drivers or added any new hardware recently?
Is the window that comes up talking about the scan from Microsoft?
You could also try to use the repair utility on your win7 install disk. Select system restore. If you have multiple choices, choose one dated earlier than when your problems started. Be careful to not go back farther than needed since system restore will, as implied, undo changes in driver and system settings - including ones you made that have been working.
Edit: sorry for the more or less duplicate suggestion. I was unaware of bobusboy replying at the same time.