Likely a device driver, although it could also be a startup application.
Boot into Safe Mode again, and look at Device Manager, View, Show hidden devices, and see if there are any yellow exclamation marks.
Have you installed motherboard drivers from CD?
Also click Start, Run, type eventvwr.msc and press Enter.
Look at System Log and Application Log to see if there are any errors. Double click error to see error information. See if that gives you a clue.