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Srvany.exe (Download mirror) allows one to install a standard application as service. The Srvany acts as a wrapper around the program and handles the service events, so when starting the service it will run the application. One uses the Instsrv.exe to register the Srvany with a service name, and then uses the registry to attach the application which should be started through Srvany.

Instsrv.exe MyService C:\Srvany.exe

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \MyService \Parameters]
Application = "C:\MyApp.exe"

Note one can control the new service from the command line using SC.EXE or NET.EXE.

Note FireDaemon is similar to Srvany with more bells and whistles, but it costs money.

More Info MS KB Q137890
More Info MS KB Q166819
More Info MS KB Q251192