I know this doesn't help you any, but I also have an LT-20 and it's a crapshoot whether Delorme's OWN software sees it when the pc powers up. There's a 50/50 chance when the pc turns on if it will pick up the lt-20 and GPS (iguidance) will work. I also use XPort.
Does the Delorme Serial Emu software icon in the system tray turn to a green sphere? If so, then Delorme's software detects it and it's got a lock. If that's green, then open Xport and make sure it's streaming the GPS data at the bottom. If it's not, then you've got a problem there with Xport (maybe it lost it's config or something) and try to re-set up Xport.
I plan on getting a new reciever with the SirfIII chipset soon and using the lt-20 as a standby (or just pitch it... it's worth THAT much )