A couple of our customers have recently had some strange issues with both Vista and Windows 7 x64 with installing the drivers for the USB Cable (which contains a Prolific PL2303 chip).
Anyway, the following link seems to have solved everyone's problems:
Which lead to:
Which lead to this:
http://www.usglobalsat.com/download/546/win_drivers.zip which is version 1013 (WHQL Win 7 x64 Version 220.127.116.11)
Now, we can't guarantee that this software isn't going to eat your computer alive from the inside because it isn't our software and we haven't tested it. However, it seems to have solved the issues.