Whichever way you go, the two will have to be networked and that involves installing a network adapter in the PC - wired or wireless.
Assuming "wireless" (and this is my standard "blurb" for networking PCs)...
To create a wireless network for Internet access and File/Printer sharing, you have essentially two options:
1) Install a Wireless Network Adapter in each PC and create an “Ad Hoc” Wireless Network. This is a network in which the two PCs talk directly with one another, rather than via a Wireless Access Point. “Internet Connection Sharing” is then enabled on the actual Internet connection in the “host” PC and the “client” PC will have Internet access provided the “host” is running and online.
To enable “Internet Connection Sharing” in Windows XP, open the Network Connections folder, right click the actual internet connection (modem connection), select Properties and then the Advanced tab. Tick the box “Allow other network users to connect through this computer’s Internet connection”.
When you enable "Internet Connection Sharing" on the Internet connection, it automatically configures the network adapter used for your "Home Network Connection" with the IP address 192.168.0.1 and subnet mask 255.255.255.0. If you have more than one adapter available for this purpose, e.g. both a "Local Area Connection" and a "Wireless Network Connection", you will have a drop down list from which to select the appropriate adapter. The "Client" PC's Wireless Network Connection should be set to get its IP address automatically, which it does by DHCP from the "Host" PC. Also, if you run the Zone Alarm firewall in your "Host" PC, the Internet Zone Security level must be reduced from High to Medium or "ICS" traffic will be blocked.
2) The preferred option for connecting more than one computer is to use a broadband Router and, with ADSL, this should be a combined “Router/ADSL Modem”. This is an “Infrastructure” wireless network because the connected PCs communicate via the router’s Wireless Access Point. Again, a wireless adapter is installed in any PC that needs to connect wirelessly, but often the main PC is close enough to the router for it to use a “wired” connection with only the remote PC connecting wirelessly.
With a router, it is the router which connects directly to the ISP and not one of the connected PCs. The router in turn allocates the IP addresses to the PCs, using one of the address ranges reserved for Local Area Networks, usually 192.168.xxx.xxx
Either of these options will additionally allow you to run File and/or Printer Sharing wirelessly across the network.
If you are running software firewalls, these need to be configured to allow access to the networked computers, which may involve adding their IP addresses in a "trusted" section.