Activation Issue - WindowsXP Pro OEM

I'm encountering a weird problem activating my copy of WindowsXP Pro
(OEM). I bought it off of Ebay and by all appearances, it's a
general OEM version, as there are no markings to indicate it's from a
big name OEM like DELL or HP.

Anyways, it installs ok. When I go to activate though, it complains
about an "unauthorized product key"

Specifically:

"This product key is unauthorized. Activation cannot be completed
until a valid product key is entered. The 25-digit product key can be
found on the Certificate of Authenticity (COA) label attached to your
computer or Windows documentation. It should be similar to the
example below. Please correct the product key below or type a valid
product key, and then press Retry.

.... (picture of a COA, spaces to re-enter the key, and some more
details about contacting tech support)

Message Number: 45090"

Anyways, I've tried this on a real machine and a Virtual Machine using
virtualPC 2007. Same results.

I've checked the key, and apparently it was correct, as it was good
enough for windows to install itself.

I've tried telephone activation, and wasn't able to activate that way,
with little in the way of explanation as to why.

Tech support and their anti-piracy people were able to confirm that
the key was legitimate and valid, but they won't tell me why I can't
activate unless I've paid for tech support services.

Has anyone ever encountered something like this?

What am I doing wrong here -- aside from buying my OS from Ebay?

-GeorgeF