c:\Windows\Microsoft.NET\Framework\vX.X.XXX Should contain the latest 32 bit version of csc.exe
c:\Windows\Microsoft.NET\Framework64\vX.X.XXX Should contain the lastest 64 bit version of csc.exe
That’s what it is for mine anyway.
BTW: You can access both by using the Visual Studio Command Line from your visual studio tools folder in your program files. It auto sets up all the paths you need to build 32 and 64 bit apps with your csc compiler.