I just fixed one like that. I reinstalled Linux and it's boot loader lets us boot to XP and linux.
I had installed Linux Ubuntu 7.10 and I didn't like it and I thought I had repaired the Windows bootloader to the original. However, it didn't repair itself, and now I am stuck after deleting the Linux partition. I cannot load into Windows XP. When I attempt to load Windows it says "Error loading operating system". Is there anything I can do to fix this problem without using the Windows XP disc itself?