When I put an Athlon XP 2000 on my motherboard, the speed is slower not faster?

Yes the 2000 is a 1.6GHz CPU..I don't know were you get the 2.0GHz from unless its over clock speed. unless there is some jumper on the mother board that allows for a higher freq. setting it should have jumped to 1.6 by itself...try looking at the user manual see if there is something you need to "set"...I know the SiS chips were known for the "jumper" ability!