Migrating ADM Files to the New ADMX Format
Group Policy has always been based on templates which define the registry-based settings that it is used to deploy. In the past, these templates were stored in ADM files, but with Windows Server 2008, Microsoft has introduced XML-based ADMX files for the storage of this information. These ADMX files provide quite a few general enhancements and security benefits over their predecessor.
This being the case, if you have the opportunity you should convert all of your old ADM files over to this new format. Microsoft provides the ADMX migrator tool, which can be downloaded here: http://www.microsoft.com/downloads/details.aspx?FamilyId=0F1EEC3D-10C4-4B5F-9625-97C2F731090C&displaylang=en.
Once you have downloaded this tool, you can convert your old ADM files by running it, and choosing the Generate ADMX from ADM option. You can then select your ADM files and perform the conversion. Once this is done you will be prompted to save the new ADMX file to a temporary location. Finally, you can import the new ADMX files into Active Directory.