Can't connect because of self assigned IP address


Several months ago the hospital where I work began offering free wireless internet access. I have been using the system without difficulty for several weeks.
Two days ago I noticed that I needed to apply a new security update for my OS 10.3.9. Since installing the update I am unable to use the wireless access to connect to the internet. The problem appears to have developed immediately after the security update was applied. I used all of the usual precautions. I repaired permissions, etc.

I have no problems connecting to the wireless system. . My Mac shows that I am connected and I am receiving an excellent signal level. But in the Network pane I get the message" You are connected to xxxx_public network but you have a self assigned IP address and may not be able to connect to the internet" (or something very similar to this). When I investigate further I also find that there is no router address listed under the "TCP/IP" tab.

Everything seems set up correctly. I have "Configure IPva4" set to "Using DHCP". One thing I have noticed is that I no longer get the screen that explains the terms of usage of the service with the "I accept" button at the bottom of the page that must be clicked to gain access to the internet. I think herein lies the solution to the problem.
Any ideas as to what may be going wrong? How can I investigate this further, or better still, how can I fix it? I can't ask for help from the computer support people because they are very hostile to all things Macintosh (really).