I think that the default for the number of processor box in the advance option of msconfig is uncheck. If you put it back to uncheck try rebooting. Then run Turboboost monitor. Run a virus fullscan and look at the monitor. It should show a fluctuation from minimum to maximum. This condition is normal.
Thank you for advice, Boyet.
I tried put it back to default. It didn't work. CPU at 2.5GHz, and 1 million Super Pi takes 16 seconds to finish.
But if I "block one processor" in msconfig, CPU will be at 3.1GHz, and 1 million Super Pi only takes 11 seconds. Weird...Huh
I have also tried re-install Windows 7, still have the problem...
p.s. i3-2100 does not support Turbo boost