Hi,

You can try deselecting Use hardware acceleration when possible in Tools (Alt + T) > Options > Advanced > General. If the problem goes away, you may have to update the graphics drivers or check if the graphics card is being cooled properly. Please see this.

It's already checked so that's not the problem. The card has a seperate fan on it plus it only shuts down when FF is operating.

So far IE been running with no shutdowns - so it's def FF.

If Windows crashes then you need to check for problems with system drivers and software that run as service like security software and make sue that you have the latest updates.

I had this same problem, searched the net, got frustrated, but finally worked it out. If I used Mozilla my computer running Xp would shut off, if I used IE I could surf the web. However flash was not working on Youtube. In the end the problem was simply that my version of Adobe Flash was not the current one, this was causing a software conflict, which in turn was shutting down my computer while running firefox. I simply went to the Adobe site, and updated to the latest Flash, and also downloaded a debugger program for IE. Since then everything is ok, Mozilla no longer turns off my computer, and IE8 works fine with Youtube.

Hope this helps, as it sure frustrated me at the time.