I had same kind of problem with my E7300.
It is clocked at 2.66Ghz and with 6x multipler it clock itself down to 1.6Ghz at idle.
And "My Computer Properties "also showed 1.6 Ghz always.
(Real temp and CPUZ Coretemp all showed 2.66Ghz).
I reinstalled XP and the problem was gone.
I think you should also reinstall your OS.
im ready for a foramt any way but ive just been puttin it off as long as posible lol
but im sure at my last format i had the same problem