This is a typical startup situation when building a new system using an AMD processor.
The original Athlon/Duron processors, and early 'Thunderbirds', ran at a 100 mhz Front Side Bus (FSB). Later 'T-Birds', Palominos, and Thoroughbreds are meant to run at a 133 mhz FSB, and newer Barton's run at a 166 mhz FSB.
However, many motherboards ship with the default FSB set to 100 mhz - this is in effect a "legacy" of the original Athlons.
For example, if you install an XP2400+ expecting it to report a 2.0 Ghz processor speed, and the BIOS or other utilities report it as an XP1800+ running at 1.5 Ghz - your FSB is defaulting to the 100 mhz setting.
Depending on your motherboard, you will either have to:
a) change a jumper from the 100 mhz setting to the 133/166 mhz setting
b) enter your system BIOS and change the Processor Speed or Front Side Bus setting from 100 to 133/166.
Consult your motherboard manual for specific details.