Yep - definately have a valid license key/progID for this. I'm using my MSDN professional key which only I have access to.
I've done a little more digging and it appears that this message occurs because a prerelease of SP1 may be installed:
http://support.microsoft.com/kb/954753What's wierd is that I am 99.9% positive that I installed the RTM version of SP1 from the MSDN download site as I usually don't install betas of service packs. However, it is I suppose possible that I tried out the pre RTM version as this isn't my main PC, rather a slightly older one I use for debugging/testing and playing odd games on.
I'm uninstalling SP1 now and going to reinstall it. HOPEFULLY I won't have to flatten this machine and start again, although the knowledgebase article seems to suggest that you need to uninstall SP1 BEFORE you get the zero hours message. Anyway, here's hoping....