I get the same error regardless of where I enter the cmd command; mIRC's /run, Start -> Run and Command Prompt all give the same error.
Code:
C:\Program Files\mIRC>cmd /C "C:\Program Files\mIRC\mirc.exe" -U "string"
'C:\Program' is not recognized as an internal or external command,
operable program or batch file.
It is likely a limitation of cmd.exe, using the $shortfn()'d filename instead should work.