FPS suddenly drop

Hey im having this frame rate problem recently
where normally it doensn't occur until the last few days.

The symptoms are tremendous drop in framerate in many games
ex. RE5 a couple of month ago i becnhmark it and acheive 60-75 FPS not bad..
on max setting with AA + blur

but now with the same setting it drop to around 20-35 badd..

surprisingly this problem also occur in crysis warhead
where i use to be able to run it on max setting with no stuttering , lagging
or delaying (only alittle bit in some epic boss fight) but now fps drop to 15-30
since the start of the game when only 1-2 enemies appear.

In l4d2 with vsync turn off and multi redering off aswell i only get about 40-70 FPS
which is strange .. i think.. as it should be much higher according to my spec

not all of my game have this issues but most have.  

here's my spec:

CPU: intel core i5 2.67
GPU: Ati 4890
Ram: 4 GB
Window: vista 32
fan olympia 700W


i have no idea what's the problem but im suspicious that the problem is burried deep.

i dont know if this is normal but.. my physical memory is at 40 percent all the time even i didn't run anything atall but maybe its vista well...

here's some things i already tried

upgrade driver to 9.11 newest!

install lastet directX

tweak some option in ATi catalyst turning off AA , etc...
(which seem to bring the fps up abit)



some one suggest that i should reinstall my window oh and metion something about the voltage but im not pro at this so..  


any idea or suggestion would be good. i would appreciate it. im not so good at this kind of thing i usually play them not fix them.. i certainly need help


Thks in advance  


(sry for teh bad EnglisH)