Two hard drives, one computer, disc one with Windows XP, remove cable from disc one, plug into disc two, load Windows XP, run XP from either disc. I don't want a dual boot system, I want a back-up computer.
I have never seen this request before, I guess you could.
Install it on the first hard disk, then remove it and then install it on the next hard disk. You can then keep switching back and forth.
However that would technically mean 2 installations of the same OS, the eula might prohibit this but I suppose you could argue that it's one computer. It should activate ok as it is the same computer... I don't know.