1. Have you tried Safe Mode logon or Last known Good Configuration?
2. If that fails if you have the XP installation CD, try repairing the boot sector from the Recovery Console comands: FIXMBR and FIXBOOT or do a Windows XP repair-installation
3. If no CD download the Hiren's Boot CD Live ISO and burn it to access the hard drive and recover files, recover the Windows XP product key, etc
4. Or from another computer, connect the H D via USB adapter to recover files, recover the Windows XP product key, etc .