Well first you should know that if your using version 1/1.1 of .NET, 64 bit is not supported. Secondly, look in C:\WINDOWS\MicrosoftNET\2.0.'something'\ and there should be a folder that says 64 bit somewhere in there.
Also, check this site out: http://discuss.joelonsoftware.com/default.asp?dotnet.12.223381.7
