When trying to install ADAM on Windows Server 2008 RC0 (aka Longhorn) Enterprise Edition, Build 6001, I faced the following error:

After doing some research, and playing around with the installation, I found the following workaround:

1.       Install ADAM on a non-Longhorn machine such as Windows Server 2003.

2.       Copy the %WINDIR%\ADAM folder that is created to the same location on your Longhorn  server

3.       Create a new registry key called HKLM\Software\Microsoft\Windows\CurrentVersion\ADAM_Shared

4.       Under this key, create a new Multi-String value called "SharedFolders"

5.       Copy the vssapi.dll file, located at %WINDIR%\System32 from the W2K3 installation, to the following destination in the Longhorn server: %WINDIR%\ADAM

6.       Now, run adaminstall.exe from your %WINDIR%\ADAM directory and follow the wizard.  Do not import any LDIF files.

7.       Finally, run ldifde.exe for each import you wanted to have in step #6.  The correct command line arguments to use are listed at the top of each .ldf file. Some examples:

%WINDIR%\adam\ldifde -i -f %WINDIR%\adam\MS-AZMan.ldf -s servername:port -k -j . -c "CN=Schema,CN=Configuration,DC=X" #schemaNamingContext

%WINDIR%\adam\ldifde -i -f %WINDIR%\adam\MS-User.ldf -s servername:port  -k -j . -c "CN=Schema,CN=Configuration,DC=X" #schemaNamingContext

%WINDIR%\adam\ldifde -i -f %WINDIR%\adam\MS-UserProxy.ldf -s servername:port  -k -j . -c "CN=Schema,CN=Configuration,DC=X" #schemaNamingContext

%WINDIR%\adam\ldifde -i -f %WINDIR%\adam\MS-InetOrgPerson.ldf -s servername:port -k -j . -c "CN=Schema,CN=Configuration,DC=X" #schemaNamingContext

8.       In order to enable the MMC “ADAM ADSI Edit” snap-in , register manually the following DLL using this command:

%WINDIR%\ADAM\regsvr32  -i  ADAM-ADSIEdit.dll

9.       Finally, add a shortcut to the MMC snap-in located at:

%WINDIR%\ADAM\ ADAM-adsiedit.msc

References:       http://dunnry.com/blog/InstallingADAMOnVista.aspx

Hope this helps!