Its Speedstep
Load your CPU with some programs and you'll see it go back up to the normal 2.4ghz
Main thing to start off is at 3.0ghz, most Q6600's with good batches can hit that easily do this in your BIOS by increasing one of these settings
on some boards increase the 266 FSB to 333 FSB, or on other boards increase the 1066 FSB to 1333FSB