Sounds like you have XP PRO (rather than home) and it is trying to log you on to the domain rather than the PC. When you get to the login screen are there boxes for "User name" and "Password"? If so, there should be a third box for "Log on to" - you may need to press "Options" to see this. Use the arrow at the end of the "Log on to" box to change to the other option. From memory - no time to reboot a PC to check - it will be something like NAME (This computer). If you can get logged on OK, next time you reboot hide the third box by pressing "Options" again.