Had some problems at a hotel client last year when the first new Vista laptops started appearing with guests and delegates. The guest wireless has no password so its as simple as its gets really. Some of the laptops failed to get an IP address allocated and therefore wouldn't connect out. The laptop would associate and then do nothing or report Limited Connectivity. Other times they would work for a short time and then appear to disconnect. Also saw what looked like a normal connection, you could ping an external IP but website site addresses and hostnames wouldn't resolve - bit strange to diagnose. Needed a Gregory House moment.
I managed to track down a couple of registry keys that helped. Thought I'd better post them now just for the record following a discussion recently about dodgy wireless connections.
First one is from Microsoft KB933340
I was reminded of the second one by Susan Bradley who pointed out Steve Riley's post.
Start a Command Prompt
(there's a cool short cut to this by shift-right-clicking a folder such as Documents)
(or use Windows-R key combo and type CMD)
then run the command
netsh int tcp set glo aut=dis