How to write an ADM file? 

It is very important to understand the ADM language for GPO Admins. At least you can fix an error with ADM. Find the below links for How to write a ADM file. Now a day ADMX/ADML is used for windows 2008.

http://en.wikipedia.org/wiki/Administrative_Template
http://technet.microsoft.com/en-us/library/cc738443(WS.10).aspx
http://www.frickelsoft.net/blog/downloads/howto_admTemplates.pdf



ADM Location: "%SystemRoot%\Inf" folder
ADMX Location "%systemroot%\PolicyDefinitions" folder


You can convert ADM to ADMX, download the below Tool.

http://www.microsoft.com/download/en/details.aspx?id=15058


Managing IE cookies through ADM
http://blogs.technet.com/b/grouppolicy/archive/2007/09/21/setting-default-registry-key-values-using-custom-adm-files-part-1-of-2.aspx


Download the Mozzila Management ADM
http://web2.tigs.com.au/blogs/agit/2007/12/01/group-policy-enabled-firefox-frontmotion/

Download Google management ADM, Its awesome.
http://www.chromium.org/administrators/policy-templates