I have an Intel Core 2 Duo E6400 that should run at 2.13 GHz but when I boot my computer, the BIOS shows a CPU speed of just 1.6 GHz (I removed the BIOS/motherboard logo screen so I can see the hardware details). CPU-Z also shows a speed of ~1.6GHz.
Since I bought my computer five years ago I have reinstalled Windows several times and can remember that BIOS showed a CPU speed of 2.13 GHz. The Windows Vista hardware rating tool gave a CPU speed score of 5.1. Last time I have installed Vista it rated the CPU at just 4.7. I then refreshed/updated my score and Vista finally rated the CPU at 5.0 (now it gives 5.0 every time I rate my computer).
Another problem that I think is connected to CPU voltage settings - sometimes my computer won't wake up from sleeping. When I press any key/click the mouse, the graphics card fan start to buzz (like when there are no drivers installed) and nothing happens. I then had to use force to restart my computer. Sometimes after that it fails to boot Windows with BIOS overclocking/voltage faild error (Only thing that helps then is to manually replug my graphics card (NVIDIA GeForce 8600 GT). This problem occurs every 30 days for no reason.
I didn't have this problem for about three months but I just noticed that my CPU speed is lower then it was so I can't be sure that those things are connected.
Intel Core 2 Duo E6400 (2.13 Ghz), Geforce 8600 GT, Motherboard asus P5LD2 SE
Here is a screenshot of my CPU-Z states: