XPS 420, Alert! CPU fan failure

Hello!

My friend have a XPS 420. Two days ago the CPU fan start run on 100% all the time. He get the message "Alert! CPU fan failure" then he starts the computer. First thing I did was clearing the CMOS but it didn't help. I have also tried to switch the CPU fan and the cage card fan.Then I did that the cage fan was running on 100% instead. Is it the motherboard that i broken or could it be something else? What could I do for find the problem or solve it?