Network connection on server shows connected, but cannot access any network resource

Has it got the DNS server installed and did it happen after a reboot?

If not ignore the rest of this, but if so note that there is a bug in W2k3 that the DNS server can grab port 4500. This stops the IPSEC service runing and blocks all network access to and from the server. The immediate fix is to restart DNS then restart the IPSEC service (or of course restart the server). A longer term solution is to add 4500 to the reserved ports in the TCP/IP service parameters.

JR

Re the DNS problem:

See http://support.microsoft.com/kb/956189

This refers to SBS, but it applies to standard W2k3 as well. I have been bitten by this several times and the fix described in the KB article works. I now routinely do this on all W2k3 installations.

There are a few things you could try, including but not limited to:

1. Does the same slowdown happen if you connect directly into the switch?
2. What kind of wiring does the building have? If it is old, this could be the issue.
3. Run wireshark to see if you have any strange traffic on a slow machine, whether that be collisions or some other packets out of the ordinary.
4. Do you have some sort of monitoring/graphing on the router? It is possible one of the connected PCs is running P2P, malware, or other software which could be using up the local or internet bandwidth?
5. If all else fails, what happens if you replace the network equipment?

I would break this problem down into parts - start at the core of the network and work your way out until you figure out what the problem is. And please share the ultimate fix! :)