The only way I know to alter the way your PC recognises the speed of the CPU is to alter either the mhz rating or the multiplier in the advanced section of the bios pages. If you had done that you would know. It's not the sort of thing you can do inadvertently.
It seems to me it might be more of a bios glitch. Try entering the bios and see if you can manually set it to 166Mhz and the multiplier at 11. This gives a 1.82Ghz clock speed which I think is correct for the Athlon 2600.