Thanks all for the replies people. Here is what I have tried so far:

(See original message for details.)

Ran a new cable from the switch to the Dell.
Ran Dell's utility to wipe the drive and set everything back to just out of the box settings.
Installed NetBUEI.
Removed the Modem card.
Disabled virous chacking.

Nothing changed. If no other computer has a file open then it's fine. If any other computer has files open then it takes 2-3 seconds to open each each shared file. The type of table buffering seems to make no difference.