Problem soved.
For those who are interrested:

The new GDS32.DLL of FB 2.0 depends on the msvcr71.dll in contrast to
the GDS32.DLL of FB 1.5. That seems to be the reason why that dll is
part of the FB2.0 package located in the Firebird\bin directory. The
problem is that msvcr71.dll is not part of a Win 2003 Server
installation (Not entirely true - it is but is located in the
C:\Windows\Microsoft.Net\Framework... directory and not System32.
So you have to copy it to System32 and the Problem is solved. I'm not
sure it the dll is part of a fresh WinXP SP2 installation. On all
systems I have looked at I found it at System32 but these were no fresh
installs.

Regards,
Frank.