Although not the same problem, I had a similar sounding problem with Vista on a wired network.
If you do ipconfig and it shows something like
192.186.2.1
0.0.0.0.
or similar then I found that remnents of Symantec were interfering with the new Zonealarm firewall and causing al sorts of Sh!t.
Make sure you only have 1 firewall type app running at once.