Before doing a full fledged repair install you could also try simply re-writing the boot sector and Master Boot Record.
Simply boot from the XP disk and load into the recovery console. When you are at the "C:\Windows>" prompt, simply type "FIXBOOT C:" and when that's finished type FIXMBR and answer yes to any prompts. Reboot and give it a try. If that doesn't work, try putting the old
RAM back in and see if that helps. If that works then you might have a defective stick.