That's not something we can help with I'm afraid, sounds like a Windows issue. If it gets that far, MBR, partition tables and the boot sector are fine. Could be a lot of things really. Maybe the system file checker can help (http://support.microsoft.com/kb/185836
) or else try repairing the operating system (using Windows setup). Could also be a 3rd party driver that's mal behaving. May also be a good idea to check the system memory.