^^^It could be Intel SpeedStep slowing it down when not in use to save power. Run CPUZ, start a game, then alt-tab back to CPUZ to see what speed you are at when under load.
It could be a setting in BIOS (restart and quickly tap DEL or F1 or whatever button it tells you will start Setup). Base Clock * CPU Multiplier = speed
It could be a jumper on the mobo. Check the manual for your motherboard. Sometimes multipliers can only be changed by physically moving little jumpers onto different pins on your motherboard. One of the first times I ever took the side panel off a computer was a result of being shipped a pre-built single core 1.4GHz AMD desktop that had a jumper on the wrong pins, setting it to 1.2GHz.
