I have a Pent III, 128 MB RAM, I took out the 128, and installed 2) 256. Computer is compatible, slots are ok?

Hi. I would start by pulling one of the 256 sticks out and try booting. Then swap for the other 256 stick and try again. If neither works, reinstall the 128 and look at the BIOS. Maybe the FSB needs to be set to auto. Good luck!