It's possible (sometimes)...but not recommended as a normal way of doing things.
Lots of variables...generally I would say that it cannot be done successfully with a "name brand" system (one by HP, Gateway, Emachines, Dell, etc.) because (generally) those systems are tied to recovery/restore disks/partitions, as opposed to a Microsoft XP install CD acquired as a separate purchase of the system hardware.
On a custom-built system (where the user has installed XP herself/himself with a copy of XP purchased as a separate item)...or one of the above-mentioned systems where the user has deleted everything installed when the system was purchased and then installed XP using a valid license/key and a Microsoft install CD...the effort can be made, but the results are not predictable or guaranteed.
Intel® Application Accelerator - Moving a Hard Drive to a New Motherboard - http://18.104.22.168/search?q=cache:cLjCO4...=clnk&gl=us
Changing a Motherboard or Moving Hard Drive - http://www.michaelstevenstech.com/moving_xp.html
Edited by hamluis, 11 October 2009 - 01:01 PM.