You can't transfer the master hard drive from one computer to another without having to reformat it or doing some serious operating system repairs. The computer probably won't even boot up into the operating system when the master hard drive from another computer is configured as its master hard drive. Most of the hardware must be perfectly identical in order for something like this to work. Switching drives and then booting up with the XP CD and performing a repair installation may allow you to keep all your old files and settings, but you'd have to update Windows again and reactivate it (Which would be a real pain to do, especially when transfering the license from one computer to another).
Edit: If, by some sheer luck, you manage to swap the hard drives and the systems do boot up into XP, their activations will be reset.
I presumed you're using XP on both systems. Activation only applies to XP and Server 2003, not any previous versions of Windows.
Edit: If, by some sheer luck, you manage to swap the hard drives and the systems do boot up into XP, their activations will be reset.
I presumed you're using XP on both systems. Activation only applies to XP and Server 2003, not any previous versions of Windows.