[PvK]

Different hard drives have different jumper settings available. The usual ones are Master, Slave, and Cable Select. The BIOS on your mother board decides based on the cable position and what the drive tells it (determined by jumper settings on the drive) which drive is the Master, and uses that drive's boot area to determine where the OS(es) it will use is(are).

In general, you should be able to set up an OS on a new drive, and then put in an old drive from an old system, setting that drive in position and/or jumpers to make it the slave, and you'll get what you want.

However, since XP has evil "authentication" software designed to give M$ an excuse to invalidate your license and make you buy Windohs again (see, it doesn't abuse its monopoly position enough to get enough sales, because its goal is as much profit as possible, so nothing is ever enough), and one excuse it uses is thinking that you may be trying to install Windohs on a new computer. Since it was installed on a computer with one hard drive, and now there's another hard drive, it may "determine" that you need to do annoying things to prove you have the right to run Windohs XP...

Personally, I'd upgrade to Win2K Pro instead of XP Pro. No evil authentication, less annoying crud, less bloat.