Some WiFi channels are prohibited in some countries. Some drivers are aware of this and allow you to choose location and let them keep you on the safe side of the law; such a driver may hide networks that are illegal according to its settings. In effect two different drivers may present you two slightly different sets of available networks.
This Wikipedia article contains some tables. For 2.4 GHz WiFi (802.11b/g/n) these are allowed channels:
- in North America: channels 1..11;
- in Japan: channels 1..13, 14 with restriction;
- elsewhere: channels 1..13.
This is just a simplified example; refer to the linked Wiki page for details.
So if your Mac OS X wireless network card driver "thinks" you're in Europe (or it doesn't care) and your Windows WiFi driver "thinks" you're in the USA, and if your own network uses channel 12 or 13 -- then you may experience these symptoms you described.
To fix this, check driver settings in Windows. Alternatively reconfigure your own WiFi to use a channel from the set 1..11; this should make it visible regardless of regional setting in any WiFi driver.