If you are running Windows 2000 or Windows XP, you should be able to use Internet Connection Sharing. You can then access the Internet from either PC (as long as the one that is physically linked to the ADSL is connected to the Internet).

Alternatively, you can buy an ADSL router. This replaces your ADSL modem and incorporates the modem, a network hub (and often some kind of firewall protection) all in one clever little box. Then, both PCs simply get cabled into the network hub part of the router and the modem part of the router services both PCs the same.

Even better than that (for some people) is that you can get wireless ADSL routers and wireless network cards for the PCs so that you get those functions without the PCs even having to be connected with cables. This is great if you have one PC in the lounge and one in an upstairs bedroom for example.