OK that makes it clearer as to the setup. What your problem is , is in the addressing of the 3 Com switch.
Most likely to be a DCHP addressing issue although it could be a IP/Subnet mask addressing issue as well or a combination of both. Since the other computers are accessing the 3 COM switch I would try first off running the Network set up Wizard again making sure that the File & Print sharing is enabled and to Auto Detect Settings. I'm betting that a static address has been set to this computer and they is incompatible with the switch.
The only potential problem that you will have to look out for is to rename the network to the correct name as the default is MSHOME which may have been altered when this was originally setup. if you are asked to reboot the computer after running the Setup Wizard you have done it properly and it then should connect on the reboot.
Lets know how you get on.
Col