All cores showing in device manager, all cores enabled in BIOS. Tried resetting CMOS using MB button as well as jumpers.
Upgraded from i5 BIOS to 2nd most recent vers, showed 1 core. Updated to very latest vers, still showing 1 core. Halp!
has a nice workup of how to do it. Basically setting the registry min and max # of cores to 0 makes windows reevaluate the processor you're using, so as long as they are all being detected properly beforehand this should fix your problem.
Now, how do I mark this as solved so I can help other people out?