kaled - 12:20 pm on Feb 27, 2010 (gmt 0)
Assuming your USB modem has an ethernet connection (most do) then the simplest solution is to order a wireless router. I use a TP-link model - they're inexpensive and mine has worked faultlessly for two years.
This has the advantage that the sharing computer can be switched off when not in use. In the first year, this might save enough electricity to pay for the router. It also includes a hardware firewall (useful, but maybe not the best).