Thank you for your post.
You definitely meet the system requirements.
We (FileMaker Technical Support) have seen this issue come up previously when trying to install as a guest of the Server. That is, make sure you are installing under the Admin account.
Also, make sure you are installing at the Server and not over the network.
If either of those two options don't work, run "msconfig" and turn off all non-Microsoft services. This provides the basic system settings and eliminates any possible conflicts.
Please keep us posted with your progress.
Ryan in tech support was able to help resolve the problem. Although we had run the OS Update everal times, MS never identified that we needed any additional updates to the OS. Therefore, we could only assume that we had everything that was available. Out of the blue on 2/25/2009 an update came in for SP1 (KB936330 from 9/29/2008) that allowed FMS10A to be installed. Why it didn't come in the previous 10+ times we checked for updates, I have no idea.
Lesson learned - don't belive everything MS tells you ;-)