Don't take Microsoft's Advice on this one. WEP is not secure.
Microsoft recently posted a column on Ad Hoc WiFi Networking on the Vista Community site and advocated using WEP if WPA was not available. This article further gave instructions on entering a 5 or 13 character WEP key without even bothering to explain the weaknesses of WEP. WEP supplies a false sense of security. In my neck of the woods, the kiddies often forego wide open networks because it's "fun and easy" to crack WEP. This is one of those backward compatibility features that I wish Microsoft had not included. Or at least given users an explanation that this is "use at great risk". The author sees the backwards compat angle but omits the red alert on security and safety side. It wasn't so long ago that MS told us all that security was job #1.
WEP is unsafe at any speed. Use WEP in a hotel and share via ICS and turn on File and Print Sharing to save money is an invitation to disaster. Just say no. Buy yourself a cheap travel router if you MUST share your connection among close personal friends (D-Link's DWL-G730AP works quite well). But do not follow the advice here unless you're interested in having your files stolen and possibly your identity as well.
I don't normally raise a red flag on Microsoft content without first having made every effort to contact them first, explaining my case, and suggesting replacement content. Be assured I've done that.
I'm a Microsoft Networking MVP, and if you know anything about me, you know that WiFi is where I live and breathe.