As others have already stated, the disks are usually tied to the specific OEM (Original Equipment Manufacturer) that produced your PC, so you can't use them for a different PC. You especially can't use those for a different version (Media Center vs. Professional) since the Product Key is tied to that particular version, that's why it's callled a 'Product' key.
If Best Buy provided an installation without disks there should have been a recovery partition on the disk as also mentioned above. If that's gone you'll need to attempt to get the recovery disks from Best Buy either for free or at a minimal media cost.
If you just have a number that someone wrote down and not an actual MS COA, then you have no real proof and might as well go buy a new version of Windows to get legal. Windows XP won't be available after June 30th I believe, so better act fast if that's what you require, otherwise Vista will be your only choice.
Never ever let anyone provide you a new PC without either a CD/DVD or at least a recovery partition, then immediately make a CD/DVD using the provided utility if it isn't supplied. Otherwise you'll end up in exactly this situation.
Bitman