If you want to actually browse DLLs in your GAC at C:\Windows\assembly\ follow the instructions below.  This register edit will disable the assembly viewer and instead let you browse files like you would any other folder and allow you to grab or copy files, etc.

1)    Use Regedit and go to HKLM\SOFTWARE\Microsoft\Fusion and add a REG_DWORD entry called “DisableCacheViewer” and set the value to 1

2)    After updating that entry you can path through to C:\Windows\assembly\ and copy or grab files, replace them, etc. 

WARNING: Always be careful and backup files before you replace them.  Also, make sure you hit CTRL+C to copy a file as dragging and dropping may actually move the file out of the folder.