The voyager 105 USB does not authenticate from the modem. It is a high speed dial up connection from your PC. Hence the software needed to set the username password.
There are ways of sharing with the current set up you have, but you would need a LAN/RJ45 port on each pc (which most PC's these days have) then you would require a x-over cable costs a couple of £'s. Then you Share you internet connection on your desktop PC. And as long as your desktop PC is on you have internet on both at the same time. (there is a bit to set up on the ipconfig on each PC, but this is just a one off)
Now I would strongly avoid the above. It is far more complecated. I would purchase a Netgear 834GT, Some packages come with a Free Wireless USB stick.PCMCIA card. Plug that into your laptop. Then use the Lan cable you get with the 834gt and plug into your PC. You have 1 off simple set up on the 834GT to do which involves entering your username/password. The rest of the defaults are good enough for an average person.
The you just connect any pc to it. Either via a cable(up to 4) or just connect via wireless. I suggest you set up some sort of encryption/access list for your wireless connection. Otherwise anyone within range could potentially connect via your ADSL. Don't let this put you off. It is a 1 off, and I sure you will find all the support you can ever dream of here.
Which ever way you decide. I am quite happy to help