I don't think the motherboard is to blame. I have the same wierd readings in my system, and I've got a gigabyte x58 board. My 12v reading sais negative 3v though, but that just proves the readings to be wrong. I'm guessing it's either the hardware monitor that has incorrect offsets, or the driver just doesn't work properly in windows 7.
There are limits in the atx specifications for how high or low the power level is permitted to be on each voltage level.
The actual limits are 5% variation in either direction for all power levels except the negative ones. They are permitted to vary 10%. And under peak load, the +12V rails are permitted up to 10% variation as well.