Excuse me for being so uninformed, but I am quite new at this. Please help me understand the USB adaptor with an external antenna jack you said to look at.
There are 3 components to make this work: Wireless USB 2.0 Adapter, pigtail coaxial connector MC or RP-SMA to N-Type Male adapter and an omnidirectional antenna with a N-type female coaxial connector. (1) The client adapter plugs into your laptop. (2) The MC or RP-SMA connector end of the pigtail plugs into the client adaptor. (3)The N-Type Male connector end of the pigtail plugs into the 2.4Ghz omnidirectional antenna. Depending upon which antenna you use, it's possible that the antenna's interface may be something other than the standard N-type female coaxial connector. If that is the case, make sure the antenna end of the pigtail matches the antenna connector.
There are many USB wireless adapters to choose from.
Once you have the hardware hooked up then, set up your computer for wireless networking in Windows 7.
A well reviewed adapter that allows the use of an external antenna is this Wireless G USB Network Adapter that works with both PCs and Macs. Drivers for Mac OS can be found here. Get the RT2571 driver. (Remember to switch to "Ad hoc" under "Network Type" in "Profile").