So I build a new Windows XP ISO with SP3 integrated, this is a Volume License editions (VOL) which only accepts Volume License Keys (VLK). When I want to install it on a clean machine, I noticed that setup prompts for a product key and it does not allow me to continue, which should be a new feature in XP sp3. OK so I fill in the VLK but it does not accept it, I try another one from a customer of mine, does not work either.

I found out that the problem is when you do the XP Sp3 slipstream process on a WIndows Vista machine, the API gives a different response and screws up the PID process. This is a bug and Microsoft knew about this in December, still the bug remained and made it all the way to RTM, nice work Microsoft.

So XP SP slipstream only on XP or 2003 machines!!!

See the dicussion thread here: Key not working after SlipStream

Source: Bink.nu