Originally Posted by hbarcat
It's my understanding (I could be wrong, don't quote me) that Microsoft keeps track of individual instances of activation using the same code. Your license agreement only allows use of one copy on one computer per code and since it's common for people to buy a copy of the OS and then install it on multiple machines and give (sell) it to their friends to install on their machines, Microsoft watches to see if a code is used more than once. If this happens, it disallows activation although you can call support and tell them you're reinstalling the OS on the same machine and they will make it work.
Yeah, I understand how that works and have been through that process when migrating other software (Office, as mentioned above) on other machines. But this copy of XP Pro had just been installed and activated once since I bought it.
In hindsight, I think I now understand what had happened here. Back in 2008, it appears I purchased an OEM version of XP instead of retail (I confess I didn't really understand that there was any difference at the time in terms of the licence). Now, 5 years down the line, Microsoft have suddenly got tough and marked as "non-genuine" any instance of XP (and maybe other products) where a bulk OEM product key does not match with the components in the PC detected by the Genuine Advantage software during the updating process. In other words, they're using the updating process to check and validate software and will flag up stuff as non-genuine even if you've had it for years.
Incidentally, the licence for Office allows the user to make two installations - one on "your main computer" and one on "a portable device". Not sure about the Windows OS licence, though.