I have ran into an issue with more than one computer that is troubling me. I fix computers in my community and have my A+ and Net+ certs, i am not a pro but I know what im doing 85-90% of the time. I ran into a problem where a computer will boot, run for a min then has some sort of power spike where the fans run like 100mph and the cpu usage jumps to 100% then fluctuates between 100 and 70% with nothing open... the fans will calm then speed up very fast again sometimes even causing the computer to shut off. i reinstalled windows completely so i know there is no spyware or virus or even unwanted processes running causing this. I even replaced the p4 processor twice with used ones of the same and slower ghz. the problem still occurs. the heatsink is not even hot so i know its not a temp issue with the cpu, all fans seem to be working. I tested the power supply with a multimeter and got good readings from the molex connectors, the northbridge however runs very hot, but i had others that ran hot with no problems. Im begenning to suspect a motherboard issue ???... the current problem is with a HP Compaq dc5000mt with 2.7ghz p4, however i had similar problem with a dell optiplex 260gx and a dell dimension.... ANY SUGGESTIONS ???
Open Task Manager and look for a process called svchost.exe. It is the process that deals with Windows updates, among other things, and has been known to murder CPU's. I have personal experience of this.
Another quick note: The reason the fans are spiking is simply because the CPU is, the fans react dynamically to the cpu temp/usage. And like Koos said, open task manager.... filter by CPU usage, under processes tab you should see something thats eating 100% CPU resources. If there isn't anything there, its likely a hardware malfunction.