I had the same problem.

My Wireless card (WG111T) was not obtaining an IP addres from the router. The card worked on other machines just not the one with the problem.

Tried all of the above but did not resolve my issue.

In the end changing the routers channel from 11 to 13 resolved the issue. (netgear DG834GT).

There are 10 other wiresless networks near me and i belive this was causing interferance.