Slow computer

I have a windows 2000 os on a 600mhz computer by dell. Its a Dell optiplex Gx100. Anyways I have a 10Gb hard drive and i am only using 3Gb. I have 256Mb of memory. My computer is always slow. Can Any one tell me why. I am in a network. But even when i disconnect from the network I am still slow.Why?
Thank you