How to start a 64-bit process from a 32-bit process

You can temporarily disable filesystem redirection around the call to Process.Start, the appropriate API's to P/Invoke are Wow64DisableWow64FsRedirection and Wow64RevertWow64FsRedirection.

Another option is to use %windir%\sysnative, which is available on Windows Vista and above.

c:\>set proc
PROCESSOR_ARCHITECTURE=x86
PROCESSOR_ARCHITEW6432=AMD64
PROCESSOR_IDENTIFIER=Intel64 Family 6 Model 70 Stepping 1, GenuineIntel
PROCESSOR_LEVEL=6
PROCESSOR_REVISION=4601

c:\>c:\windows\sysnative\cmd
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

c:\>set proc
PROCESSOR_ARCHITECTURE=AMD64
PROCESSOR_IDENTIFIER=Intel64 Family 6 Model 70 Stepping 1, GenuineIntel
PROCESSOR_LEVEL=6
PROCESSOR_REVISION=4601

c:\>

What you do is you use %windir%\sysnative to resolve 64-bit CMD.EXE and then you launch your other 64-bit program via "/c" command line option.