I assume that you are saying that all is fine with XP and not fine with Vista, since I've not read any earlier posts on your problem yet.
Apparently there are a lot of problems with USB converters working only occasionally or not at all with Vista, and it requires a converter that repeats rather than just reroutes the EIA signals, and proper RTS/CTS flow control must be supported in the converter.
In the Callsign Software Yahoo group yesterday, there was a posting indicating that the RT Systems model RTS-03 USB-serial converter is supposed to work just fine with Vista. I think the item was bought from AES.