Sometimes if you install a new memory card and it is not a match to the existing memory cards, you will need to use the XP install disk to do a repair install (not reformat) of the operating system. This does not erase your personal files.
This configures the new memory.
Also if you plug the memory into the wrong slots that can cause a problem, check the motherboard manual for the correct slots to use.
Or you could just have bad memory stick.