changed processors from amd athlon xp 1600+ to a 2800+ and it says unknown cpu running @ 1.6 GHZ not 2.25?

Answer 1

If you didn't change the motherboard as well, probably the current BIOS doesn't recognize the new processor. The only thing you can do is to update the BIOS (from the mobo's producer's website). First though check if the processor is connected accordingly (you could test it on another computer if you have this option)

Answer 2

Check your multipliers and bus speeds in your bios. Try resetting your cmos.

Answer 3

Your present motherboard might not be able to support your new CPU.