You need to fix the master boot sector of the affected hard disk.
You can do this using your original XP installation CD.
Boot from it.
When prompted, press R to repair a Windows XP installation.
If repairing a host with multiple operating systems (which is the case), select the appropriate one (XP) from the menu. If you have only one operating system, enter 1 to select it.
After booting, you need to provide the administrator password for this specific installation.
To fix the Master Boot Record, please type:
Review all the warning messages and at the end press Y fallowed by ENTER to fix the MBR.
Than type exit to reboot. After this your XP installation should continue without problems.
You *will* have problems to load all other installed Operating Systems. This is because fixing the MBR will replace the information stored about the previous bootloader which were in charge of those.
There are various possibilities to make your system know that there are other OSes installed. I don’t know if you are confident any of them, but if not I can point you PLOP Boot manager or EasyBoot CD. They are both easy to use and will get the job done.