Acquire Windows XP License Keys Legally

Retail copies of XP can be resold with no problem. OEM copies are tied to the machine (specifically, the motherboard) and therefore non-resaleable after install. Also, you want to have physical possession of the sticker the key is on.

Newegg is still selling XP Pro licenses, but they are very expensive at $440 for a 3-pack or $140 for a single license.

Have you tried eBay or Craigslist for old retail copies of Windows XP?

"eBay should be a good way for most people to get it. You don't have to worry about counterfeits if you pay attention to their feedback scores and reviews (nobody is going to blow a 10,000 rep for $50), and make sure to open a dispute in time if it does turn out to be counterfeit to get your money back. Plus, if you find someone in your area, you can opt for local-pickup and meet them in person to examine it yourself and save on shipping.

Another option is online classifieds. That way, you can meet the seller in person and examine it as with a local-pickup. Many countries have Kijiji (renamed to eBay Classifieds in the USA) and others that don't usually have some sort of local counterpart. It is indispensable because it greatly helps to reduce waste by making it easier for people to buy and sell locally. I highly recommend it for general use."

"If you already have XP installed as the host, I have been told you can legally isntall and run up to 4 virtual instances with the same key. Something I was told at a Microsoft TechEd Conference.

From: Windows 7 licensing and virtual machines clarified

if you have Volume Licensing, an SA and the Windows 7 Professional
Upgrade License (Volume Licensing Upgrade License) or Windows 7
Enterprise then you can run 4 virtual copies of Windows 7 on a server
or desktop"