cable modem

Go out and buy a Netgear or Linksys G router with a 4-port switch. Usually can find one on sale for $30 and $40 resp. (may involve a rebate). Don't bother with trying to use the USB port on it. You could try using the desktop temporarily via ICS as a gateway for the laptop, but I wouldn't - the router gives you a lot more flexibility and safety.

First, the modem itself may only support one port at a time.

Second, even if it allowed both at the same time, most cable companies only allow one IP address at a time, at least not without an extra fee.

As others posted, get a basic broadband router. They are cheap and usually include at least a 4-port switch.

Finally, unless it's USB2 HiSpeed (not to be confused with USB2 FullSpeed, which is the old USB1 HiSpeed), you may not be able to get the full bandwidth out of it anyways.

You're in luck this week:

Office Depot has a Linksys WRT54GC for $20 and no rebate !!!!!!!

They're probably going to be hard to find though - they're out online, but you may find on locally.

Staples had a similar deal with rebate (rebate may end today or soon - not sure).

If you don't find a deal today, there will be other decent deals next week most likely.