Power Mac locks when connected to network - OK otherwise

Hi,

I have recently bought a silver i5 Mac Pro which I knew had a faulty network adaptor/card. It did not cost me much as a result but I am having a v odd issue with it.

Although I use macs at work, this is the 1st one I have owned so I am not familiar with the more technical aspects, such as system preferences, the command line etc.

I bought a retail Snow Leopard dvd from the Apple store and used it to scrub the partitions and reinstall Snow leopard 10.6.3. (it already had SL installed but I wanted a nice clean mac without the possibility of any nasty surprises). The install worked fine and I was able to upgrade to 10.6.8 by downloading the updater from the Apple store onto a usb from my PC.

I then downloaded a wifi driver from the Tenda web as I want to use a Tenda W522U usb wireless dongle.

The drivers installed Ok and I can connect to my wifi router.

However the mac randomly locks up if I do. It works fine if it is not connected to my network, but I get odd lockups and the spinning beachball if I am. For example I just got a lock up clicking the ? in the system prefs -> keyboard window. I always get lock ups if i try to run Safari, Itunes, or Software Update.

Once again I have no problems if I am not connected to the network.

I uninstalled the driver and hunted around system preferences and found the network option.

Of things were odd up until now they now get very odd.

The network window is telling me that the ethernet is connected! Yet I have nothing plugged into the ethernet port and do not have the Tenda usb dongle connected, or indeed its drivers installed.

If I plug a cable into the ethernet port (which I have been told does not work) then I get the same random lock-ups.

I am baffled.

Can someone suggest how to try and find out what is going on? I have used Linux in the past and believe OSX is based on a Linux like system so although I do not know what to enter on the command line I am sort of familiar with using a command line, just not a mac one.

I hope someone can help me solve this weird problem.

Huge thanks in advance.