a free option

Most of the time your ISP will charge you $5 extra each month if you want two IP addresses.

Here is an option which cost much less:

If you just want to browse web with the other PC, you can buy a HUB, connect both of your PC to the HUB, connect one of the PC (call PC A) to the ADSL modem. Then you can download a proxy server and install it on PC A.

On PC B, from IE [tools]-->options--> connecitons-->Lan Setting-->Proxy Server (set it to PC A's internal IP address), now PC A has two IP address, to find out which IP address to use, run-->[cmd]-->ipconfig, use the one associated with Ethenet Card.

A hub cost about $8-$10, two RJ45 cables cost $5 you can get it for free anywehre.

The disadaventage is you can only do HTTP/FTP from PC B. if you want to Telnet out or Exceed out not possible with this option. these services are used mostly by people who works from home .

hope it helps.