If you have a retail "single user" licence, you can move the windows installation from one pc to another, but can only use it on one PC at a time. Using it on more than one PC is not legal, you wouldn't be able to activate the second installation and it would not update. So in order to use windows xp on your new pc you'd have to uninstall it from the old one first(even then you might get problems with the activation and you'd have to contact Microsoft).
If you have a OEM version, you can't use it on another PC unless your hardware is the same (not likely

).
What's more, installing windows xp on pc's that come with vista isn't always easy. First off, you'd probably need to load the drivers for your SATA disk - for that you'd need a floopy drive or a slipstreamed windows xp cd. And once the installation is completed, you'd usually have to find and manually install different drivers for hardware not supported in the xp cd.
So, to sum up, it can certainly be done, but it can be tricky depending on your level of knowledge and the particulars of the model.
