Thanks for the reply, 'iccaros'. I got it sorted out. Wow, am I relieved!
Yes, I have 3 physical disks on the desktop. I was attempting to install Ubuntu 9.10 via UNetBootin on the same disk as Windows XP, such that Windows occupied 120GB and Ubuntu occupied the remaining 40GB. At the 'Advanced Options' screen just before finalizing the installation, I saw that I could change the installation location of the bootloader, but I left it at /dev/sda. In retrospect, I should have probably changed that to the Ubuntu /boot partition because I remember doing so on my Vista laptop so as to avoid tampering with the Vista bootloader. On that occasion I just used EasyBCD from within Windows to point the Vista menu entry to the relevant Linux boot partition. Unfortunately, I can't get EasyBCD to run in XP; perhaps an older version will.
Anyway, after no success with the Windows Recovery Console, I booted UBCD > Seagate DiskWizard and used 'Update MBR' to fix the Seagate disk. And it worked! Rebooting took me into Windows XP!
This was quite a learning experience. I was looking into the Super Grub Disk tool, but that seemed a little too complicated for me, still being new to Linux. Also discovered that one of my disks needs error-checking thanks to a background disk utility found in Ubuntu 9.10.