|It really depends on the way you bought the original XP. I'd suspect that the xp came as part of an OEM install. It is not easy to move them and usually against the eula.|
The second part if the laptop. A vista laptop is more unlikely to be a good choice to install xp. Too many drivers may be needed.
Best solution is to return the laptop back to OEM state using disks or partitions. If you have enough ram you may be able to then run a virtual machine to then run xp.
It may be legal to then take your PC and make it into a virtual machine with a P2V application. With that one image you could move it back and forth if you then put some linux with the same virtual machine on it.
You can have only one copy installed at one time is the normal rule. Some tech's can have a test copy in some countries and you may be able to preserve the original disks in a copy.
1/3 of highway deaths are caused by drunks. The rest are by people who can't drive any better than a drunk.