To bad you are using Windows, to my knowledge the are no free tools that put the card into RF monitoring mode so you can find cloaked SSIDs (not broadcasting their SSID). Kismet can detect cloaked SSIDs and can be ran from the Knoppix Linux boot CD pretty easy.

As for USB cards, I don’t know how well supported they are in Netstumbler or Kismet, I’d stick with a PCMCIA card with a Hermes or Prism chipset (them may be 802.11B, but they will detect 802.11G, at least in mixed mode).