First, try what jc2it suggested. I always forget about that myself.

Second, are the files on the slow computer a lot larger than the ones on the other computers? It is possible that the person using the bad computer is just using large files.

If that's not the case then try network packet sniffing. Network packet sniffers are excellent diagnostic tools. Use Ethereal on the client and see what kind of network dialog is going on. You may find that you have a lot of duplicate packets, retransmitted packets, and out of order packets. This could mean that your NIC is broken.

Anyway whatever you see will give you some idea about the network side of the problem.

Ethereal works on Windows, Linux, and Unix. You can download it for free at

www.ethereal.com

You can restrict the display to traffic to/from the client by using the filter host where is the actual ip address of the client. Actually this would work on a computer on the same LAN as the problem computer.

You can see the packets in real time by using the Options menu windows and select "Update display in real time". The default is to save packets to a file and not display them until you "playback" the file.