Assuming your mothrboard is capable of overclocking, there will likely be a menu option in there. It sounds like your system is running with a slower FSB speed than the chip expects, which is causing it to run slower.
The Athlon XP 2400+ normally runs on a 133MHz FSB (266MT/s effective speed), and has a clock multiplier of 15.
133MHzx15 = 1995MHz, or around 2GHz. It sounds like yours is only running at 100MHz, causing it to run at
100MHzx15 = 1500MHz.
If that is the case, try increasing the FSB speed from 100(200) to 133(266). I'm not sure whether your board will display the bus speed in terms of actual or (effective) speed, hence both being shown.