How do I boot to a DOS prompt? Hold down F8 while booting? I can try
that ...

If it's 2000 or XP, I'm not sure there is a DOS option. I wouldn't rely on
the O.S. that's causing problems in any case. Try going here to create a
boot disk:

http://www.bootdisk.com/bootdisk.htm

The Windows 95 Version B is one I've used before.

And no, I don't overclock, and I didn't change any of the multipliers or
FSB's when I switched CPU's.

The multipliers are built into the CPU, so you couldn't change that. What
about a BIOS update? Did you update your BIOS in between switching CPU?
Just wondered. You could reset your BIOS settings to the default. I don't
think it's anything to do with Windows, which is unusual. :-)

~ Adrian ~