I upgraded a machine at work from 2000 Pro to XP Pro, and now I cannot connect to the network. I get a "media state.....media disconnected" message in the Command Prompt. Any good pointers, I have a few ideas myself, but more is always better.

“Media Disconnected Means” Your PC is not able to connect to a network drive [mapped Drive] may be you are not in the network or you are not correctly configured your IP ADDRESS or you are using a deferent username to login(other than your windows 2000 username)

First go to Start, Control Panel, System, click the Hardware tab and click Device Manager. Check to make sure there isn't a yellow exclamation point next to your network card.
Go to Start, Run, type CMD, press enter. In the command prompt type IPCONFIG and press enter. Do you have an IP address listed? Do you have a gateway listed?
If so then type PING and then the ip address listed for your gateway. If you get returns then you have a connect to the network.
Now go to Start, My Computer, and click on My Network Places in the left hand panel. Click on Entire Network on the left, double click Windows Network, What workgroups do you see? Can you open any work groups and see PCs in them?

Try these out and let me know the results and I can give you more specific advice on solving the issue.