Q: Why does Netstumbler close/shutdown/crash after only a few seconds?
A: Because the card you are using is not on the list of cards that work. Get a supported card, problems go away.
Q: But it worked fine before, it just suddenly quit.
A: As the great philosopher Kurtis Blow once said, "These are the breaks". Cards not on the list MAY work by using the NDIS 5.1 driver. Performance is not guaranteed. Someone with an identical card and identical system may never experience a crash, while you may never see Netstumbler work. Keep this in mind when you search and see that some nipplehead bragging about how his Brand-X-not-on-the-list card works perfectly. In all likelyhood, he will be back asking why his card quit later. If it's not on the list, it's not guaranteed to work. Some unsupported cards work better than others. Before buying a card, search to see how well it performed for others. Or skip all the drama and buy an Orinoco or rebranded equivalent.
Q: My Orinoco card crashes/closes/shuts down Netstumbler.
A: The README/List of cards that work isn't very clear on WHICH Orinoco card to buy, because when it was written, there was only 1 type. Now there are different combos with different WEP strength. The card you want is referred to as the "Classic" B-only Orinoco. These can easily be found on eBay for next to nothing or from Fab-Corp. For more info, refer to this thread.