Plz computer experts accept this challenge. See details?

Hi everyone. I am suffering from a very weird problem. I have 4 Ram slots and I have one 128 mb ram installed in one slot. But the system properties shows only 96 mb ram installed. But when i lok at the system information it shows full 128 mb ram. Also crucial site shows 128 ram. My computer is a P4 but runs slower than any P# comp. Also when i am using any program the CPU Usage drops down and the application stops respondin. PLZZZZZZ HELP EXPERTS IN GETTING WITH THIS COMPUTER. I WIIL RATE 5 STARS TO ThE BEst anSWER>