I7-2600 Running only 3 cores (fixed)

I have a friend with a CPU problem. He uses a gateway model no. DX4860-EF32P. His i7-2600 has recently been running only on three cores. However all four and four virtual cores appear on the device manager processor list, but the fourth core is absent everywhere else. Anyone have any idea on why this run away core disappeared?

Edit (Fixed): Friend just realized that he had set his computer to boot up on only 3 cores through msconfig. We reset the properties in there and everything is all normal again.