In your two examples, this is what REG sees as it's arguments:
- ADD
- HKLM\Software\My Co
- /v
- Data
- /t
- REG_SZ
- /d
- 100
- /f
However, since to the Windows "shell" being very, very,
very limited and not allowing nested quotes, this is what your ssh command looks like:
- ADM_B
- REG
- ADD
- HKLM\Software\My
- Co
- /v
- cat
- /t
- REG_SZ
- /d
- 100
- /f
But since you've apparently got Cygwin installed anyways, use the Bash shell that's installed with it, since this shell knows about different types of quotes and how to nest them.