[DllImport("kernel32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool IsWow64Process(
     [In] Microsoft.Win32.SafeHandles.SafeHandleZeroOrMinusOneIsInvalid hProcess,        
     [Out, MarshalAs(UnmanagedType.Bool)] out bool wow64Process
     );
[DllImport("kernel32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool IsWow64Process([In] IntPtr processHandle,
     [Out, MarshalAs(UnmanagedType.Bool)] out bool wow64Process);

//...

isWow64 = false;
if ((System.Environment.OSVersion.Version.Major == 5 && System.Environment.OSVersion.Version.Minor >= 1) ||
      System.Environment.OSVersion.Version.Major > 5)
{
     SafeProcessHandle processHandle = GetProcessHandle((uint)System.Diagnostics.Process.GetCurrentProcess().Id);
     bool retVal;
     if (!NativeMethods.IsWow64Process(processHandle, out retVal))
     {
     throw new Win32Exception(Marshal.GetLastWin32Error());
     }
     isWow64 = retVal;
}