Obviously you'd get a lot of "found new hardware" dialog boxes.
And, if you're using XP or Vista your activation would be invalidated and you would most likely have to call Microsoft within 30 days to reactivate it and keep using it.
Overall there are many reasons why this is not advisable. For one thing, your install might want a different hardware abstraction layer (HAL) than the one it came from, and might ultimately prove unstable.
That said, I did move an NT 4.0/Exchange 5.5 server from one box to another by doing a drive transplant and we kept on using that system for years though it did have some intermittent instability issues that may have been tied to that.
It sure would make life easy if Windows would be able to boot off external drives on relatively random hardware (like Linux or OS X are capable of doing), but it can't in its native state.
B