sonic wall TZ strange behavior

I am a tech on small networks and normally do Watchgaurd boxes.  About 6 years ago i put out a Sonic Wall TZ router with some basic settings (mainly port forwarding).  Nothing much.  I almost don't want to pose this question but...

My client has one slow PC that I've worked on to get back into shape.  After working on it, it slowed down again.  I then told them time to re-image(they don't want to buy a new machine).  They had another tech in that posed the following to them...It is the Sonic Wall that has made that user's PC slow and that router needs to be changed out.  I am not a guru but can't figure out this logic.  The unit has been fine for about 6 years.  All other PCs run with proper speed.  

Does anyone know of a condition that can cause a user on a PC to be singled out by the Sonic Wall router?  
Jon ConantAsked:
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Ravi AgrawalConnect With a Mentor Commented:
No way, you are right!

It is some misconfiguration on the slow running PC. Check it out for sypware / malware / NIC installation issues or other hardware issues. If not, reimage the system and if you don't trust the image reformat, then reinstall from scratch. If it is really slow, tools are there to diagnose the cause of slowness. What Operating system is installed on the PC in question?

If other PCs connected to the router behave the same way, then sure something wrong with the router. It is the basic process of elimination and diagnosis we are talking about.

I have written this in light of all what you have mentioned in your question. If you left something out, then I may be wrong.

Ravi AgrawalCommented:
It could just be that one particular NIC port of the router (in our case, I am assuming it to be the one connected to the slow PC) has gone faulty. It happens, Have you tried swapping the Cable into another Port of the router?

Jon ConantAuthor Commented:
Thanks, No other PCs behaved badly...they are identical to the slow one as they are a set of 3 identical dell machines and the rest of the PCs (running W/XP) are OK also.

Regards, Jon
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