Program or Network? Tracking down the real culprit

Hi,

I am trying to find out whether a friend's work computer problem is caused by a Program or the Network, and I could do with any ideas that I might have missed.
:-)

At the moment I have just stuck with the one workstation that seems to be having the problems.

Situation:
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* 4 Windows XP Pro workstations connected as a Workgroup.
* Each can access a company Business program held on one of the computers ('the Server')
* The Business program is invoked from an MSDOS shortcut from the desktop. This causes the program to open up in an MS DOS window on the workstation.
* Initially a faulty hard drive together with corruption was thought to be the culprit.
* A complete clean reinstall from scratch was done. Afterwards it is faster than before the reinstall and I would say this is now normal/typical performance such as Internet speeds, wordprocessing.
* I used Sandra to analyse the computer afterwards - and all looks good.
* However, when running the business program (across the network) it is intermittently slow at times to the point of freezing the program.
* A quick call (and importantly free help!) to the makers of the Business program got a reply that they thought the network was the cause of the problem. (Without jumping to conclusions as to the cause, it uses NetG**r devices plugged into the mains). These devices then connect by ethernet cable to the computers.

Questions and Thoughts
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How can I rule out the cause?
The fact that it is occuring at one workstation (the most used apart from the server) makes me think it is not the Business program - since I would have expected all workstations to exhibit the same behaviour.

If the Internet is working ok, then I guess the network link is persistent.

Maybe I need to install some free software to measure what happens between the two computers over an 8 hour period?

I have tried swapping the NetG**r adapter and the problem remains.

My only other thought is to bring in 'an ordinary hub' and wire the suspect computer, server separately and see what happens...

Anyone got any ideas or suggestions please?

Thanks in advance.

Pete