My system is decreasing in performance.

I am facing a weird problem now a day. My system is gradually decreasing in performance due to high disk activity from system process and perform.exe.
Whenever the high disk activity occurs, my RAM usage drops from approximately 1.6GB to just 600MB. I don not know the reason for this error but I am facing this since last about two weeks and could not find any solution yet.
Also this high hard disk activity problem will occur anytime whether I am playing some game, browsing or doing nothing.
