If they were on separate disks, you might be able to boot Windows XP since the MBR on the Xp installed disk would still exist. The boot loader for Ubuntu was on the disk that it was booting from. You might be able to get away with changing the boot disk in the BIOS.
Since you're posting, I'm assuming that you're trying to boot XP unsuccessfully without the Ubuntu disk as the main boot disk in teh BIOS? If that's the case, the MBR on the XP disk will likely need to be rebuilt. You should be able to boot to your XP install disk, enter the recovery section and use 'fixmbr'. If that doesn't work, try going back into the recovery section and doing 'fixboot'.
All this was under the assumption that you weren't talking about using the same disk with different partitions....
