You will need a Virtual PC package to run it in XP mode. With premium you cannot use the Virtual PC XP-mode supplied free for W7 but you may be able to use some of the Virtual PC products available. If you have XP disks with the product key then you have a valid license and can use with any virtual PC software.
Oracle's VirtualBox is free with registration
If your PC was bought commercially, like from Dell or Gateway etc then it may still have an XP license code on the top, side or back, but you will need to find the XP software to download and install; with the legal license, you are not violating any laws downloading it if you can find it.