Ok here ya go. All thats wrong is you Motherboard is just set the defaut safe speed of 266 which in the bios will be 133 amd's are doubled. you have 2600XP thats a barton core 333Mhz. So just go into the bios and change the FSB to 166Mhz. (that will be 333Mhz) That is the correct setting for a Barton core. And that IS NOT overclocked it will be running at the speed its suppose to be.
Hope that helps if not post again.
Hope that helps if not post again.