Need some help

I’m working on a HP Pavilion 7851, well it have by default a 128 Mb SDRAM PC133, so i wanna to upgrade it, so i bougth a 256 Mb SDRAM PC133 it didn’t work, so i put it a 128 Mb SDRAM PC100, it work fine.

Suddenly the computer reboot twice, and the thirth time the machine died on me…

What should i do?

I turn on the computer, but not even the bios screen shows, i’ve removed (the new, the old, both of them) memory dimms but the computer doesn’t do anything.