If this is an OEM computer i.e. it came with XP preinstalled and has a holographic sticker on it with a key, you have 2 options- one, simply order the recovery disk from the OEM vendor, no need to buy from ebay. Two, get hold of an XP disk of the same edition and preferrably the same service pack, install and activate with the sticker key.
FYI, the recovery disk from ebay etc. will work only if its the right edition and SP. Much better to order it from the oem.
But if the computer is not OEM, none of the above applies.