Sometimes when you add memory, if the card is not an exact match to what is already installed, you can get problems.
Doing a repair install of XP will usually set up the configuration and cure the problem.
Look in this XP forum, fourth post from the top, for the directions to do a repair install.
I have seen some systems fail from adding just 1 memory stick, until the OS was repair installed. Good idea to back up all your personal files before trying anything else.