Normally people would leave a drive in the computer with XP on it and then have a second disc which can be moved from computer to computer with there files on it.
Because of the way XP was designed you can't swap hardware in and out of your computer any more. The installation/setup/cd key that you had to phone up microsoft for is based on the hardware in your system. Therefore if you change to much hardware it simply stops working.
You can change one or two peices of hardware without problems however if you simply swap the drive to a second computer all of the hardware changes and thus won't work. If you change more than a few pieces then you ahve to once again phone up Microsoft for a new code that won't work on the old system.
Finally I am also not sure if using the same operating system on two computers from the same drive is legal. The normal definition is one copy of the O/S one machine. Granted you have only installed it once but I am really not sure.