WSUS Client Updates

I have re-installed Windows Update Service 3.1 on our servers. My sychronization has been successful, but I am unable to get any of the machines (2000, xp, vista) to respond to the client. I do see all machines on my domain and have added them to 2 computer groups. Is there a new client that I need to push down to my machines? Also, My GPO is very simple and only tells Windows Update to point to my WSUS server, so I am assuming something is out of sync with my client and server.