This is easy to do, but keep in mind that you are implementing a briged connection between two buildings. A bridge lets you keep all PC's on the same subnet, which will simplify your life supporting most network applications.
You need a separate WiFi "router" (really an AP -Access Point) in each building. Set each AP to "bridge mode", if they have this mode. This is similar to "ad hoc" mode, but with special bridging software to make the connection more robust, and to allow you to use the same sub-net in both buildings. Not all wireless routers have a bridge mode. You may be able to accomplish the same thing by setting both AP's to ad-hoc on the same channel.
First, set up everything using default settings, with exceptions below, on a clear channel. Available channel numbers are 1, 6, and 11 if you are in North America, and appropriate channels for your country if you are not. Set up each AP bridge individually, using static IP addresses that are in your main building's subnet. Turn OFF the DHCP server in both bridges. You want the DHCP to come from your primary Internet router, not the AP's. In true bridge mode, the "gateway" for the PC's in the remote building will simply be the main Internet router's IP address. However, if you have to fake it with AP's that won't really "bridge", you will probably have to generate DHCP from the remote AP, and use it as the gateway for the PC's in that building.
After you get things running you need to turn on security. don't do this until you have connectivity; it will make your life miserable trying to solve multiple problems simultaneously. (Ping is a quick way to make sure you have connectivity, or you can just try to connect to the Internet.)
For security, do these steps one at a time: Put the MAC address of each AP bridge in the other AP's access list. Check to make sure it still works. Now, turn on high encryption (128bit or 168bit if you have it). WPA modes are best for WiFi, as the older WEP stuff is fairly easy to break into. Now, check to make sure it still works.
Remember that cable has a lot of loss at these frequencies, so use the shortest possible lengths and the best (and biggest) cable you can.
Can't do a block diagram, but this is generally what you will have:
Internet:
-:DSL/T1 Router, subnet of 192.168.1.0/24, gateway ...1
(DHCP on)
-:AP Bridge 1, static IP 192.168.1.200 (arbitrary)
(DHCP off, WPA on after initial testing)
----:< antenna to Bridge 2 :----
:-AP Bridge 2, static IP 192.168.1.201 (arbitrary)
(DHCP off, WPA on after initial testing)
:-Your PC's, which get DHCP from DSL/T1 router
and use 192.168.1.1 as the gateway. PC's in
this building will be in the 192.168.1 network
and pick up a network from the Internet router
Issues: you may have to manually set DNS, as XP often will not pick it up from some DSL routers. If you can Ping a router at the other end of your Internet connection (the ISP's DNS router, by IP address, is a good test), you have connectivity, and just need to take care of the DNS problem manually.
Good luck!
J TRU