With my little experience with BC, I have noticed that far away signals that don't normally show up in OS X do regularly show up in XP's wireless networks; however; the signals are so weak that XP can't connect to them regardless. XP is more sensitive in that regard or maybe just more lenient on what it chooses to display ["Hey! Here are some nice wireless connections a hundred yards away that you can't connect to!"].
Sooooo yeah. Maybe it has to do with how your router sends out the signal and how Windows/OS X picks it up. You might be able to change the router settings on how it emitts its signal which may or may not help. Like pulse/burst/directional or whatever else there might be. I have no idea