Technically it is possible but not without a lot of work and annoyances from what I have read on it. Perhaps someone has found an easy way to do it?

I have a similar question.
I bought a new laptop which came with Windows Vista Home Premium.
I removed the hard drive from my old computer which was running on XP.
I used an adaptor purchased from Frys to hook up the old hard drive to the new laptop (through a USB port).
The new computer does recognize the old hard drive (which has XP in it as well as numerous files)but I am not able to open the files("Access Denied"). I go through through all the steps of "File Sharing", "gaining control" etc... No luck
An easier way would be to boot my new Vista Laptop through the old laptop drive which has XP installed. h
There must be a way. This would be the best of both worlds: Using the external old hard drive to run with XP and using the internal hard drive with Vista.