Hi Becky,
You can use that copy of Windows on your PC as long as it's not OEM (ie came pre-installed with the computer from the manfactuer), because they are meant for use with one computer only. If it's a retail copy of XP, then you can install it on any system you so wish, as long as it's not installed on more than one machine at a time.
If you wanted to install a retail copy of Windows on multiple PC's at the same time, you'd need to purchase a seperate license for each machine.
Hope that clears things up for you.
Pgh