The most likely cause without digging into it further is that there is a bad driver sucking up 100% of however many CPUs it's using.
You can try to get some more details out of the task manager (such as the actual DLL name that is using the CPU) or try downloading Process Explorer (Process Explorer) and see if that will tell you. If you can get the name of the actual driver file causing the problem, then you can try uninstalling that piece of hardware and installing a different driver for it.
Edit: If you download Process Explorer, run it, double click on the "system" process. Then select the "Threads" tab (Click ok on any error dlg that comes up).
You will see a list of running threads in "system" and the CPU usage of each. Each thread also lists the file XXX.dll or XXX.sys etc that the thread is executing for. That file name can then be traced to a driver and hopefully a piece of specific hardware you can disable or install a different driver for.