Confirmation ID invalid

I recently applied SP1 to my laptop (running XP pro) and
was for some reason prompted to re-activate the OS.

It's a valid copy so I went through the process without
any qualms but I now find that I can't complete the
activation process.

Either using the automatic phone system or talking to
a "real person" I get to the point where the confirmation
ID has been entered and the "next" button has been
enabled. I click it and get the message "The confirmation
ID is invalid."

I've tried various things to get round this including most
recently re-installing XP as an upgrade. During this
process I get a message saying that the "windows logon
software" has been changed and is incompatible with the
upgrade. I'm not aware of having done this myself but I'm
not the first owner of the machine so I don't necessarily
know the whole story.

Does anyone know:

1. What the "confirmation ID" message really means.
2. What the "logon software" message really means - i.e.
what does XP think has changed (msgina.dll appears to be
OK).
3. Are the two messages connected.
4. Where do I go from here (other than a compete re-
install which I am trying to avoid).

Thanks

Paul