I have performed the following actions:

C:\Program Files\Firebird\Firebird_2_5\bin>instsvc sto
Service "Firebird Guardian - DefaultInstance" successfully stopped.

C:\Program Files\Firebird\Firebird_2_5\bin>instsvc r
Service "Firebird Guardian - DefaultInstance" successfully deleted.
Service "Firebird Server - DefaultInstance" successfully deleted.

C:\Program Files\Firebird\Firebird_2_5\bin>instsvc install -s -c -g -name FB21
Service "Firebird Guardian - FB21" successfully created.
Service "Firebird Server - FB21" successfully created.

C:\Program Files\Firebird\Firebird_2_5\bin>instreg i
Firebird has been successfully installed in the registry

C:\Program Files\Firebird\Firebird_2_5\bin>instsvc sta -name FB21
Service failed to complete its startup sequence.

When trying to start it from the Task Manager it complains about a missing file - where is the path of this file (presumably the fbserver.exe) saved and does it use the same one for all instances.
Bennie