This could have various reasons. Could be a hardware malfunction (had about the same). I also noticed that desktops having multiple network cards (wireless counts to) do tend to have more problems. If possible I would swap the network card with one of the 2000 machines.
Check if the settings of irq etc in bios is about the same setup as al the rest.
I also had some lock ups with old hub hardware and the wrong selection of the driver on windows xp. eh DRIVER is important but also the selection of half and full duplex, and the cabling.
If you have a windows xp running for a long time, and the user installs about everything I would also recommend to re-install.