If you have the XP ce (not the recovery disks) then boot with the cd and choose the recovery console. At the Prompt type in
and hit enter. When its finished type in
and hit enter
after its finished try rebooting.
If it doesnt then there is a third option in recovery console FIXMBR but here is Microsofts warnin on using this
"Use this command to repair the MBR of the boot partition. In the command syntax, device name is an optional device name that specifies the device that requires a new MBR. Use this command if a virus has damaged the MBR and Windows cannot start.
Warning This command can damage your partition tables if a virus is present or if a hardware problem exists. If you use this command, you may create inaccessible partitions. We recommend that you run antivirus software before you use this command."