Problem solved, I know. But doesn't an OS needs a certain amount of space to boot. I think XP needs at
least 1.5Gb of free space. If you have an unpartitioned HD and a shedload of files all on the same drive as the OS, Windows will stop working.
The OP gave a clue:
My 4-5 y/o desktop decided to come up with the message "unable to load operating system"
the other day.
It was running Windows XP from an 80GB hdd that was almost full.
The solution is to delete all those vids/pics/music files/.tmp/whatever files.
The OS will then load.