A friend recently called up, stuck on trying to get their brand-spanking new Apple Airport Extreme router working wirelessly with a Windows XP computer (he uses Macs, his wife uses Windows - imagine the arguments between them about that! 
). The router connects fine to his Mac wirelessly, but for some reason the Windows XP computer could not see the Airport at all, yet can see all the neighbours' networks. Clearly it wasn't a range issue.
After trying a few things, and googling, I found that it was because Apple Airport Extreme routers are, by default, set to wireless channel 13. I found this quite odd, given channel 13 is prohibited in the US (where only channels 1 to 11 can be used). It can be used elsewhere though (including Australia), but why not just ship it on channel 6 or 11 like all other router manufacturers so you don't have to change it for certain regions?
Anyway, because the US only allows channels 1 to 11, it was likely that on the Windows computer, the region on the wireless card was also set to the US, hence the Airport transmitting on channel 13 could not be detected. It was set to the US probably because the person who installed the drivers couldn't be bothered changing it from the default and didn't know the consequences.
So the solution? Rather than checking and changing the wireless region on all computers, just change the Airport's channel to 1, 6 or 11 (the numerical gap is designed to minimise interference). To do this, fire up the Airport utility on Mac or Windows, find Manual Settings, and under Wireless there should be an option to change the wireless channel. I don't know the exact steps as I helped him over the phone, but it shouldn't be too hard to find. See below for more detailed instructions.
If your Mac was connected before this change, you may have to tell it to reconnect so it can recognise the change in wireless channel.
Hopefully this helps someone before they start launching into a Windows sucks tirade 
UPDATE (11/6/2007): There seems to be a lot of people stumbling on to this page looking for instructions on how to change the channel, so I did some research. I can't guarantee these instructions work because I don't have one, but here goes:
- Open up the Airport Utility, located in the,
Mac OSX: Applications -> Utilities folder,
Windows: Start -> All Programs -> AirPort folder.
- If you have more than one base station, select the base station you want to configure. Click rescan if you can't see it there.
- In the Base Station menu, select the Manual Setup option. Enter your Airport password if asked.
- Click the Airport toolbar button, then click on the Wireless tab.
- Change the channel to 1, 6 or 11 (the blue box below), and make sure the radio mode is n as well as b and g, given most laptops do not have n support yet (the red box below).
Screenshot from the Designing Airport Extreme 802.11n Networks manual.
- Click Update, watch Airport reset itself, and all should be well.
Hope this makes it clearer.