Brian Livingston has filed an exhaustive article about Windows and Wi-Fi security at Windows Secrets Newsletter: Brian interviewed me for the article, which was entertaining as we sorted out a lot of the issues about WEP and WPA, and his article is an incredibly clear set of the best advice I've seen on the topic. Every Windows user employing Wi-Fi needs to read his article and make sure you've done what he suggests.
One of the conclusions I reached while being interviewed by Brian is that WEP truly is dead. There are WEP algorithms in updated firmware that eschew the weak initialization vectors that make it trivial for a cracker with free, simple software to extract a WEP key. But there's no way a consumer could possibly know whether a given device had strong or weak IVs. Further, a single network device with weak IVs makes a WEP-protected network vulnerable.
So I have to agree with Brian: there's no reliable way for the average person to use WEP and not have vulnerability that can exploited in seconds to minutes. If you don't live near anyone else at all, WEP (or no security) may still be viable. Otherwise, WPA or WPA2, and WPA Enterprise or WPA2 Enterprise is even better.