It's far from that easy. And varies with the OS in question.
Here's one about moving XP -> http://www.michaelstevenstech.com/moving_xp.html
Covers most of the issues, including the OEM warning.
Bob
What do I delete in the registry to move a drive from one chipset to another?
I remember an old Computer Shopper magazine had the answer to this, but I am unable to find/remember what that was
TIA

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic