|Maybe you should look at this in a basic, but logical, manner. Microsoft retains ownership of all of their software, end users are merely licensed to use the software. When you buy a computer from a Microsoft licensed vendor, like Dell, with Windows XP on it, a license to use Windows XP for the life of the computer comes with the purchase. It's a single license for using Windows XP on a single computer. It does tranfer with the computer if the computer is resold. If you go to someplace like Best Buy and buy a retail box of Windows XP, it comes with a license for one user to use on one computer, you DO NOT buy the software - you buy the license to use it. If your Dell OptiPlex GX 150 did not come with a license for Windows XP, and you have not purchased a license for using Windows XP on it, then it would be "illegal" to be using Windows XP on that computer.|
Now, here's a scenario; a computer is brought with Windows XP on it, so the owner of that computer has a license to use Windows XP on that one computer. But, the computer stops working for some reason and is sold or discarded without the OS on it (hard drive was stripped). The owner also has another computer and uses the backup or re-installation disk from the discarded computer to put Windows XP on this 2nd computer. Is that legal? Microsoft would most probably say NO, but if such an arguement actually went to trial (which would NEVER happen), it is very probable that the court would consider that Fair Use because the owner had paid for a license to use Windows XP on the original computer and, due to that computer breaking down, was only able to exercise the use of his legally purchased license by putting the OS on another computer. Which is why Microsoft would NEVER allow such a case to go before a judge.
If your running a business and the BSA does an audit on you, you will have to present the licenses for all the software you are using on your business servers and workstations. The BSA does not have prove you don't have a license, you have to prove you do. Without documentation to show otherwise, it is considered that the software is unlicensed. The service tag on your Dell computers will provide documentation showing what OS was on the computer when originally shipped and that provides sufficient proof of license. But, your Dell OptiPlex GX 150 would show NT 4.0 and you would have to present other documentation to show there was a license to be using XP on the machine.