Why doest this 512mb sdram work?

my OLD computer only has 128mb sdram pc 100 in the first slot.. i wnat to install ubuntu(256mb required) so i bougha 512mb stick.. the computer doest recognize the new RAM in any way.. whether its all by itself in slot 1 or 2 or if its together witht eh old ram switching slots as well.. and yes my new(if you wanna call it new) is sdram pc100.. everything the same..