"mIRC is not a valid Win32 Application"
It IS a Win32 application, but obviously it is not a VALID one to be ran as a service.
As a (possible) workaround, you may be able to create a valid exe which runs mIRC, and then load that exe as a service. I've never done this, so I can't give any direction on how to make the exe.
-genius_at_work