weird problem -new memory wont boot

bought new memory.
removed old memory stick and put in new stick now system wont boot. i tried to put back the old memory stick and system still wont boot. i used the same slots as before so i dont understand whats going on.

what the heck did i do wrong?

my system is
asus p5nsli
old ram (2x512mb ddr2) corsair
new ram 1x1gb ddr2) corsair

windows xp

did you try resetting your bios ?