DTC install fails on Win 2003 R2 x64 SP2 from x86 process

`SOFTWARE\Wow6432Node\Microsoft\MSDTC\Security` doesn't exist on a fully patched Server 2003 R2 x64
so registry key was always NULL. May also fix #1690

Tested on
- 2003 R2 x86 SP2 with all critical updates
- 2003 R2 x64 SP2 with all critical updates
- 2012 R2