Eight cores showing, so it does seem very likely that it's one core running flat-out. Personally I prefer the other view, One Graph All CPUs - there you should see a nice flat line showing the total processor load, rather than watching how Windows spreads the load among the cores (which I really couldn't care less about - although I notice that it's only loading four of the cores, not sharing the load onto the 'virtual' ones).
I also enable Show Kernel Times, which (as I understand it) will at least give some indication of when the CPU is occupied with disk access and other 'low-level' activity.
