FileMaker updated their trial download this Tuesday to have a full installer for FileMaker Server that included the 10.0v2 changes. Previously you most likely downloaded the 10.0v1 and then applied the v2 patch.
I havent installed the new full install yet, but with the old 10.0v1 with v2 patch it was fine on Server 2003.
If you want the old v1 install I am more than sure that it can still be found.
Spoke with FM Tech (again). Solution: Open the unstuffed FileMaker Server 10 folder. Open the Files Folder. Run Setup.exe. within the Files folder.
The setup.exe file in the FileMaker Server 10 folder creates the error.