DDR 512Mb Problem

The computer is running slow, taking long to open and close programs, but runs normal once programs are loaded. The HP cpu XP with stock 512 RAM only shows 488RAM in system properties. I thought it was a bad DDR chip I replaced both, then one at a time, in different slots. It displays less than what is in the slots. IS this a corrupt driver problem? Is it a CPU mother board problem? Has any seen such a problem?