These problems are usually related to a virus. Your System or Registry files could be infected. Do you have the windows CD? Or is it pirated? If you have the CD you can rested the Master Boot Record, and start it up manually. Otherwise you may have to reset windows completely, in which case you'll lose everything that you haven't backed up on an external drive. I think you can try booting it from the set up menu (pressing F 12 after start up, I think) and from there you should be able to move forward. The problem is getting it to start. Once it starts, run a full system scan and catch the virus, then you'll be set.