Only two cores out of four showing?

Previously my computer worked fine with my Intel Core 2 Quad processor. Recently I noticed that in task manager and msconfig and even in hardware manager it was only recognising two of the cores. In device manager all four cores are showing and working correctly, but in task manager it is only allowing me to see two of them when previously I could see them all. I have noticed the slowdown that this is causing my computer. Also I noticed that in msconfig it had automatically set itself to boot using "both" of the cores, when previously I had the option to use all four. How can I recover the other two cores ?

Update

It's ok now. I shut down the computer in the hope that I might get lucky after restarting, and I kind of did. I tinkered around the bios settings but didn't save any changes. While rebooting I got the blue screen of death (shock horror ) and then when I restarted it again it did a system recovery and now I have use of all four cores, hopefully permanently. I'm sorry I can't explain it more fully for anyone else who might get this problem, but I'm just not sure what happened.

D247 said:

I bet you tried to use the feature in MsConfig where you pick the number of cores to use on bootup.

Yes I did try to use that, but even while selecting the number of cores to use for bootup it would only show two in the list - the other two were not present. A good system restore did the trick though, even though it removed some of my applications