prime95 and bad ram

ive been having problems with my pc crashing alot so i put it back to default settings no overclock ran 2 prime95 ran stable for 24+ hours but after still would crash all the time so i formatted my hd and reinstalled windows xp pro the pc still crashed all the time so finally i started to trade out componets and found out my the ram i installed 2 months ago was bad replaced the ram pc work perfectly why did prime95 not show errors if the ram was bad

what version of prime are you using ?

also, what version of memtest were you using ?