Hi steven_frost, It seems you missed my questions! ;o)
The little information I have on USB and modems is that your USB ports will deliver 500m/amps of power with a +/- of 5 m/amps. The Voyager modem is a good quality modem but at times will require the full 500 or slightly more.
If your USB is only providing 495 m/amps this will lead to your connection dropping. One way round this is to buy a Powered hub. Connect the modem to the hub and the hub to one of your existing USB ports. This will provide full power to the modem at all times and give you an extra 3 ports to use as well. Cost approximately £20.00.
Using a BT Voyager 100 with AOL running XP had a problem at first ie couldn't open pages or the connection would drop it was my Preventon firewall that caused the problem changed to Zone alarm pro works fine now couldn't say if this is the fault in your case though.