%windir%\Microsoft.NET\assembly\
%windir%\Microsoft.NET\assembly\GAC_32
for assemblies targeting 32-bit.
(On 64-bit windows, will run under WOW64).
%windir%\Microsoft.NET\assembly\GAC_64
for assemblies targeting 64-bit.
(On 32-bit windows, will not run).
%windir%\Microsoft.NET\assembly\GAC_MSIL
for assemblies targeting 'Any' platform.
(Will run on any platform that supports .NET).