If a HD with a bootable OS is installed into a PC running Linux then
on the next boot it will detect the new OS and add it to the boot
loader automatically.

If that drive is then removed it will still appear in the boot loader
but a simple command run from terminal will remove it :-)

So yes, adding an existing WinXP HD is as easy as that, but bear in
mind what was said about using a WinXP HD that wasn't installed on the
hardware you intend to use it on - you may get lucky but more often
than not you get a failure to boot or a re-activation prompt due to
the change. This may well be accepted on a phone call to MicroSoft but
there's still no guarantee that WinXP will actually work on the new
hardware.

If you want WinXP in Ubuntu on hardware that hasn't had WinXP on it
before then consider Virtual Box as an alternative. Most things work
fine that way and also doesn't mean you have to re-boot to go into
WinXP. VBox allows running WinXP as a program *within* Ubuntu :-)

K> Are you saying that I can have XP on another drive all together and
K> Ubuntu will see that drive and install it in the boot menu?