inny,
As Matuka & grindkid have pointed out, you cannot connect the XBox to a USB modem.
What you can do, however, is leave your USB ADSL modem plugged into your PC's USB port, as normal, & put a network card into your PC & connect that to the XBox. Then all you need to do is set up Internet Connection Sharing (ICS) on the PC, & you'll be able to use XBL.
Cheap (just costs a tenner for a network card) & easy to set-up. Although the drawback with ICS is that the PC must be turned on to use the XBox online.
See here:
http://www.xbox.com/en-GB/live/start/connect/sharedpc/
Or, you could get a new ADSL modem, one which has ethernet (or at least has both USB & ethernet).
Argos sell a Binatone ADSL modem with USB & ethernet ports. You would only be able to use one thing online at a time though - you couldn't run the PC & the XBox online at the same time.
Another alternative is to buy yourself a combined ADSL modem & router. The PC & the XBOx would both plug into this, & you would be able to use each online at the same time (or on their own - no more need to have the PC turned on to use XBox Live like with ICS). I don't use ADSL, but I have heard that the Speedtouch 530 is quite good for this. Getting a combined ADSL modem & router is probably a better idea than simply buying a new modem, as it lets you use PC & XBox at the same time, saves swapping leads, etc.
EDIT (Tez): apparently, the Binatone ADSL modem which Argos sell is actually an ADSL modem *and* router (despite what the Argos site says). ADSL modem with built-in NAT router with a DHCP server. Only has two ports (as aready said above), but that's enough if you only want to connect a PC (using USB) & an XBox (using the ethernet port).