I think that the OP wanted to know if it is possible to make a service exe that can be run on the command-line as well as a service.

Yes, this can be accomplished using the Perl Development Kit from Activestate. With PerlSvc, the service exe can be run in "interactive" mode if used on the command-line and also as a service.

But of course if the OP wanted to create a voodoo keylogger with a hidden process, he could use Win32::Process::Hide