Richard, here's a picture and a 1000 words



Go and buy a wireless ADSL router.
Ditch your USB modem.
Get a wireless PCMCIA or USB thing for your laptop.
Your pc has a network card by the looks of things, so that will be ok, unless you want to physically locate your PC away from where your ADSL socket is.
If that's what you want to do, get a wireless USB dongle for the desktop too.

The Wireless router plugs into the wall (where your USB modem is connected at the moment).
It will have four or so network ports at the back, you can plug your desktop pc into that if you don't want wireless between desktop and wirelss router.
The wireless router will then act as a central point for all your network activity, be it connect to the internet, or, share files between the two PC's.

I would recommend that you buy the router and usb/pcmcia wireless stuff of the same make to avoid any compatibility/range issues.

PS: forgot to say, in the picture above, the desktop pc can be connected to the router either directly, or wirelessly, there is no need to do both at the same time
Good luck