Well, I just tried starting the server via batch file. The file contained the single comand "start calibre-server --with-library=<insert_library_path_here>" and resided in the C:\ root directory. I added it as a task to be started on system boot. After I pointed the task scheduler to the .bat file, it asked me with what user credentials the task should be started, which may be the crucial point- the credentials given have to allow access to both the calibre-server.exe as well as the Calibre library directory. Your normal user ought to work fine.
I was, I have to say, for whatever reason unable to connect to the computer after it reached the logon screen (neither ping nor the calibre server, I suspect the firewall is at fault, I don't have the time now to fiddle with this further). But after I logged on, I was able to connect to the server via network, and the task manager showed the server as a running task. Personally, I'd classify that as a proof of concept and advise you to try and find out what is going wrong on your end. Posting actual error messages, if there are some, might help.