Can you even boot to Safe Mode by tapping F8 as soon as the boot process begins? If you can get it to boot into safe mode then try accessing the Event Logs to see if you can see what error message is noted there.

To check why your system had a failure, check the Event logs for any error messages. You can find the Event Viewer in the Administrative Tools, or by typing eventvwr in the Open box from the Run command.

Have you cleaned out the inside of the computer itself? Could even be a build up of lint, etc., on the inside of the tower.