Recipe for disaster:
- Start installing Ubuntu 10.4 off the CD image next to already existing OS installation;
- Encounter a CD or hard drive read/write error;
- Watch installation process bail out unceremoniously;
- End up with an unbootable system.
Ubuntu 10.4 LTS installer, for reasons unfathomable, installs a fresh boot loader (which it, of course, can't yet initialize) before the end of the installation.
Either spend time fiddling with GRUB and/or Super Grub Disk, or boot from Windows XP installation disk (if that's what you had there before), get into Recovery Console and execute FIXMBR - this will render the system bootable again.
Caveat: the Recovery Console will ask you for the administrator password. It is possible that you'd be staring at that in disbelief, for you never had to specify it (apparently, this is the case with really old pre-SP2 installation disks) - just tap Enter.