That does seem really weird, my PC bottlenecked Bad Company 2 and both cores of my processor were maxed out all of the time. My next guess would be an optimisation issue, make sure you have all of the latest drivers installed. Aside from that I'm clueless.
My first thought was maybe your processor was a bottleneck, I get a similar thing with Bad Company 2, but if you said you can run that and crysis ok then I am not sure. Check the usage of your processor anyway, just in case.