Well, if you mean move a hard drive from one computer to another without having to re-install windows , it can be somewhat tricky. And you really shouldn't do it. But out of laziness, I did it for 4 different computers (First a P90, then a K6-450, then a Duron 1Ghz, then a Athlon 3400).
Basically, the last time you run windows on the computer you are taking the hard drive out of, delete all the hardware devices via the System thing in the control panel. Then shut it down and put in the new computer.
When you fire it up, hopefully it will detect all the new hardware and install drivers for that and then you can make sure all the old drivers are gone. But you run the risk of not getting them all and having problems. Or it not working at all. But I was lucky.
And that rather battered copy of Win 98 still used less memory than Windows XP....(though it was a bit unstable)