There is a difference in the licensing with the OEM versions. If you buy the retail full or upgrade versions of winxp and make enough hardware changes to trigger the activation nonsense where you have to call microsoft, they'll happily reactivate your installation.
If you have the OEM version and change enough hardware, microsoft will probably tell you too bad, because the OEM version is only licensed for use on the original hardware it was installed on so if you upgrade it too much, it's not the original system so the OEM winxp license isn't good anymore.
I think it's BS but that's how it is. I know someone who had that happen and he was pissed. He bought a complete system (parts he put together) and installed windows, activated it, and got all the updates. A week later, the PSU smoked taking out almost every component in the computer. He sent it all back but instead of getting the exact same parts back, he made some changes. When he tried to reinstall windows, it wouldn't activate and microsoft told him "sorry, can't help you, this is between you and your system manufacturer."
