Well this is weird...
The "Active Processor Cores" showing 1,2, and All is normal. I don't think you can run an i7 with only 3 cores active is why.
However, the fact that windows still doesn't use the other 2 cores is mystifying. At this point I'd call up the people you bought the i7 from and see if they've heard of this happening before, you might need to get a replacement.
I might be missing something really obvious here, it seems ludicrous that a chip would fail like that. But with a motherboard that supports the cpu type, updated to the latest bios, with win7 x64...I really don't know what else it could be.
I'll just look up the documentation for your bios, make sure there's no setting there that's being overlooked.
edit: the only thing I could find is that the motherboard doesn't officially support windows 7. Do you have a copy of XP or Vista laying around? You don't even need to have a key, just install a trial version and see if all the cores are detected. I believe it's legal to download a copy of xp, so long as you don't get a key for it.