Legally, no. The Windows operating system are licensed to use on one PC only unless you have volume licensing. But, that doesn't prevent people from resuing it on other PCs
. If you have the original Windows XP CD, you should be able to install it for your new setup. I would just get the Vista OS for system builders. They're around $100. That way, you have a newer operating system.
In regards to sound card, this is highly optional. You can probably save even more money by not even having it. It just depends on how picky you are when it comes to the quality of sound. Motherboard build in sound is pretty good now these days. Should you have to get one, Creative sound cards do support Vista: