You likely won't. You'll have lots of driver issues, as the hardware is not the same.
In the event its Windows XP, then it will not boot up probably at all, and even if it does, it will require reactivation, to function. Not to mention the removal and re-installation of all drivers for the hardware in the new machine.
Why do yo want to do this? Does the new machine not have a boot drive already?
If you want to get at your files, you can install the drive as a slave, and use it as storage.
If you want to boot from the other drive, you safest, easiest and less painful bet is to reinstall windows on the drive, once its plugged into the new machine.
Otherwise you are just looking for a world of trouble.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.