I wouldn't buy it on Ebay. An individual selling a Microsoft Op system on Ebay is likely selling a used copy and you would have no way to know until it was too late, (and therefore as worthless as what you now have installed).
You really need to buy it from a legitimate Microsoft software dealer (or directly from Microsoft) and you can find legitimate dealers on the net that offer discounted prices on legitimate software. One such authorized dealer is Newegg.com which has XP Home (OEM Version Full Install, SP2) for about $94. Another is tigerdirect.com. You may find it for less from other legitimate dealers.
I personally wouldn't buy XP Home as its product life cycle (which includes support and Critical Updates) is scheduled to end much sooner than XP Pro, therefore I would spend a few dollars more and get XP Pro instead. Additionally, Pro offers far better security and many other features that Home doesn't have.
If you wish, you can buy an additional license directly from Microsoft and they will assist you in validating it on your computer so you do not have to reinstall the op system should you wish to keep XP Home and not get Pro instead.
What you did by installing Windows XP (or any other software for that matter although some aps - but not any for op systems to my knowledge- are valid on a second computer owned by the same party - read the EULAs involved) on a second computer without having a license for it is called piracy and you broke Federal law by doing so. Hopefully you have learned a valuable lesson.
Edited by Enthusiast, 04 February 2006 - 10:51 PM.