I have the same issue. I set the BIOS options to prevent the power saving features to slow down the CPU, but still the PC runs slowly.
Sometimes in the BIOS the current speed is the regular one (1.8 GHz for my Turion TL-56), but when logging into windows it goes back down to 800 Mhz.
Perhaps important thing if it's any help is that CPU-Z reads the speed as 1.8 GHz, but rates HyperTransport at the mentioned 800 MHz. DarkDemon could you check what CPU-Z says to you? Other people on the internet saw a low multiplier, but generally the processor model wasn't relevant (I read about TL-50 all the way up to TL-60. Everyone is stuck at 800 MHz)
Please help us!
PS: already tried uprgrading and downgrading BIOS, and powering up with only AC power plugged (no battery inserted)