What can cause Windows XP to reject a valid product key?

I bought a new motherboard last week. Windows 7 is running well, so now it's time to get my Windows XP running.

I can't install direct from the original Windows XP Home CD (retail version) because it crashes, possibly because it's only SP1, also because of problems with AHCI. So I made a slipstream Windows XP disk using nlite version 1.4.9.3 (most recent version - I've checked now).

The basis of the slipstream disk is the Windows XP Home SP1 retail disk. The only changes I applied using nlite were to change it to SP3 and to include the AHCI/RAID drivers for the motherboard. This resolved the crashing issues, and the install now progresses as far as entering the product key. But it rejects the product key, saying it's not valid.

Motherboard is an Asus M5A97 Evo R2.0, and the AHCI/RAID driver was on the CD (though the easiest way to find it was to create a RAID driver "floppy disk" using an IMDISK virtual drive - the hassles of using legacy stuff on modern machines).

Note - this is based on an original Windows XP Home retail CD, and using the valid Windows XP Home retail product key supplied with that CD. The key is valid, I have re-entered several times, and I have double-checked with the yellow "sticker" on the original cardboard folder with the documentation. I've installed from the original CD many times before without problem, and I'm sure this isn't the first time I've made a slipstream disk from it. And I'm definitely using a retail XP home product key with what should be a retail XP home slipstream disk.

In case nlite had somehow converted a retail CD into an OEM cd, though, I also checked with two valid Windows XP Home OEM licenses - and they were both rejected too.

Finally, this is during the installation of Windows XP. The machine has all network connectivity turned off. Even if I've upset Microsoft by using this XP license on too many different PCs over the years, the installer should have no way of contacting Microsoft to know that - that should be a problem for activation, not for the initial entry of the product key during installation.

So - can anyone suggest a reason why Windows XP would reject a valid product key and what I might try to resolve the problem?

Update

I've made a slipstream CD from an OEM Windows XP Home SP2 disk, with SP3 and the AHCI/RAID drivers integrated as before, and using a valid OEM product key. That worked fine. I have backup disk images so I don't need to go through all this again next time.

I'm still curious, so I may experiment with a slipstream of the retail disk with AHCI/RAID but left as SP1, or integrating SP2 rather than SP3. I haven't found a way to check whether my product key has been blacklisted, but I'll try that again too.

I won't be finished with this for probably a few days.