you might start by trying to boot up using your XP disc. Put it in the drive and start it up. You will have to switch it on first then put disc in and switch off. Then start it up again. you should get the option of repairing windows or resetting it to factory settings. This will however loose all your data so hopefully you are backed up. Also be sure you have all your discs to put your programs back in.
Thats all I can thjink of but hope it helps.