I'm a little slow. I have USB ports, but what would the adapter be called?At a minimum a wireless adapter, either PCI or USB.
The reason I said "at a minimum" is because I assume that signal is coming from some utility pole outside and you may have to connect to that signal at a window, not wherever you want to use the desktop. That can get a little complicated, especially for somebody just beginning a Wi-Fi experience.
Thanks a bunch John. You really helped me understand what I need.I'd probably consider a standard 802.11g PCI wireless adapter an a hi-gain antenna that you can place near a window. If the signal is pretty weak, a directional one might be better. For extreme range, one of the external really hi-gain directional models might be best.
Hawking Tech has a number of products that will help you increase your wireless range. The root page is Hawking Hi-Gain™ WiFi Range Extending Products.
Some of the more interesting products are this Hawking [HSB2] Hi-Gain WiFi Signal Booster, which can be used on either end of a wireless connection to boost the signal power.
Another way to increase your signal strength is by the use of hi-gain antennas. You can choose from omni-directional or directional models, here are a some examples.
Hawking [HAI7SIP] Hi-Gain 7dBi Omni-Directional Antenna
Hawking [HAI15SC] Hi-Gain 15dBi Corner Antenna
[HAO14SD] Outdoor Hi-Gain 14dBi Directional Antenna Kit
For 802.11n applications, this ZyXEL ANT1106 6db omni-directional antenna can be used.
For really long range outdoor applications, this 24dB parabolic WiFi Antenna may be a good choice.
If you have a wireless adapter that doesn't have provisions for an external antenna, one adapter that I've had good luck with is a Rosewill RNX-G1 USB Wireless Adapter. It's feature is that is has a removable antenna and will accommodate replacement antennas.
This is just a sample of available products, many people have hi-gain antennas with similar specifications, but I haven't seen any other suppliers of signal boosters.