|Are you able to boot into Linux? If yes can you see your XP hard drive or partition.|
When your system was working:
How were you switching between Linux and XP
If you were using a bootloader which one was it?
Was it NTLDR, GRUB, LILO or some other boot manager?
Are you using two separate drives for Linux and XP or 2 separate partitions?
If separate hard drives, which one is master and which one the slave?
Are they both on the same IDE port, or are they SATA disks.
If separate partitions how is the disk partitioned?
Have you checked your BIOS to ensure that your hard disk nominated as the boot disk is the right one?
Have you checked your BIOS to see whether the XP disk is being detected or not? Your post #2 seems to suggest that this may be the problem.
☺ When everything else fails, read the instructions.