The computer is a desktop, and I would like to know which is better, the small adapters that plug into the back of the computer or the desktop adapters like the Linksys models that sit on a desktop. The computers are about 20 feet from each other through a wall. The router is an AT&T model which is a combination modem/router. Is there anything I should look for in particular?

Also, I have an IMac with an airport card which is currently on this home network in that room. I am replacing it with the above mentioned Windows XP based machine. Once I have installed the adapter, do I need to install the AT&T software to configure the connection for the internet on the XP based machine, or, like the Apple, will XP automatically find the signal? Or is there something else I need to do to establish the connection in XP?...

Thanks in advance...

For a desktop, I would recommend a PCI Wireless Adaptor, but a USB Wireless Adaptor (one which plug in the back of your PC) will be just fine.

Windows may be able to detect the Device and then it should be able to let you connect to the network/internet. If this is not the case then you will need the drivers, which would be in the manufacturer's website or a CD that came with the device.
Once installed it should find the signal just fine

On the note that you are chucking out your iMac: DON'T.
I don't even know why you would want to they are more secure than a Windows PC, when it come to the internet.

No way am I chucking out the IMac. It's getting a new home a little further from the router. Amazing to me how far ahead of its time the IMac was. This one is a 99 with the airport card. Awesome machine with 768 MB RAM. The processor is old school, but the machine is GREAT!
I don't know if it ever will, but I'm sort of hoping it becomes fashionable to upgrade Macs someday. Maybe someone will take some of the great old cases and really bring them into the next age, so to speak. The cases alone are great.
Thanks for the information. Found a little adapter for about $20.00, but I gotta wait for it to ship...Ah the price of saving $$$...