|"Could the problem be a driver that worked well in service pack 2, but need to be updated to work with service pack 3? Or is it a hardware problem?"|
Not a driver problem if it's happening before the Windows log-in screen. Windows drivers don't get loaded until after you log in.
Anyway, SP3 doesn't need different device drivers compared to SP2. If you go to any manufacturer's driver download page, it doesn't ask you which service pack you've got.
If Windows starts and runs okay, no error messages during boot time, i wouldn't worry about it.