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Your Athlon XP1800+ should be running at 11.5*133.33=1533Mhz
If you have your system set to run at 100Mhz FSB then it will be 1150Mhz.
Make sure your motherboard is set to run at 133Mhz FSB. That is most often the problem when you get a cpu that POSTs at a lower than rated speed. There is probably a jumper on your motherboard to set this and/or a BIOS setting.
Another thing to do is to make sure you are running the newest BIOS version for your motherboard as well as resetting the CMOS (another jumper on your motherboard). Also tell your BIOS to Load Setup Defaults, or Load Failsafe Defaults if the previous solutions don't solve your problem.
Good luck!
Gaidin
Edit: When your BIOS is saying that you have a "1415+" Athlon XP that is NOT saying that you have a 1.415Ghz CPU. It's saying that you have an Athlon XP chip that's roughly equivalent in "performance" to an original Athlon Thunderbird running at 1.415Ghz. The Athlon XP rating scheme can be pretty confusing since it's, well, kind of silly.