You don't have to get too wild and crazy. You can use Native QTP functionality to help out a bit.
I use InputBox to allow users of my scripts to define the environment at runtime. From the input they enter, you can define other paths (e.g. setting the URL, etc.). Essentially it will pop-up an InputBox allowing users to enter valid info. From that info, you can decide how to handle with Select statements - or if you only have two paths, you can get away with If/Else logic.
Env = InputBox("Enter the Test Location: PBGDesk, PBGVM, or MNZ")
Env = UCase(Env)
Select Case Env
Case "PBGDESK" Env = Env
Case "PBGVM" Env = Env
Case "MNZ" Env = Env
Case Else Env = msgbox "Bad Test Location Selected - Ending Script"