Won’t boot on CPU upgrade

I have a machine running W2K with a Pentium II (233MHZ) on an Aopen AX6LC motherboard. No real problems, but I wanted a bit more speed. I picked up a Pentium III Celeron (566) and a slot 1 adapter and swapped it out. Everything looked okay as I booted up, but then it hung up and stalled (on the second splash screen). I swapped it back, no probs. I put the Celeron back in and tried from scratch, doing a clean install of W2K. Everything was okay during the install, but it stalled at the same point when it came time to boot. The thing is, this chip works fine with dos, W95 and ME (on the same machine), but for some reason, will not boot 2000! Any ideas?