Retrieve DLLs from GAC

Assemblies are found in C:\Windows\Assembly

You can retrieve the DLLs using Command Line.
If you "dir C:\Windows\Assembly" you will notice that there is more than meets the eye. You can browse the directory structure for x32 and x64 DLLs, etc.

Where your Sharepoint feature files are stored

C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\FEATURES -- look for your feature files here

C:\WINDOWS\assembly -- contains the assembly file (retrieve Public Key Token and/or uninstall feature here)