Sudden slodowns while playing games

Here we go:
I'm having some problems with sudden inexplanable slowdowns in games:
My system is
ASUS P5LD2 SE Motherboard
Intel Pentium D930 CPU @ 3ghz, 2x2mb l2 cache
4GB DDR2 667Mhz (2x512mb elixir,1x1gb buffalo,1x2gb kingmax)
ASUS EN7600GS (Silent Cooling Geforce 7600GS - 256mb, 400/800mhz)
Maxtor 200GB 7200rpm, 16 mb cache
Rexpower PL-450 power supply


For example.
I tried playing crysis on medium details. Works fine , until everything just slows down. Not only the graphics, but the sound, everything.
I thought it was because my pc was a little bit too old to play crysis on mediums.

I tried Flight Simulator X - i get regular slowdowns (only the graphics here) from 25-30fps to 5-6fps.

I even tried The Sims Castaway Stories. Same thing. Regular slowdowns.
I tried Everquest 2 (very high). It runs just fine , but same story. Regular slowdowns.

And they all do the same thing: if i quickly ctrl-alt-delete , switch to the processes tab , i see that they are fully using 1 core of my cpu. (that means 50% of the whole cpu).

It does this in firefox sometimes too. I'ts been happening for a month now and i have no idea what's causing it. Tried to reinstall OS's , nothing. Tried lots of stuff, including older video card drivers .
I'm on vista. I disabled vista's search indexing and preloading of applications into memory.

I can't explain it. The Witcher , Gothic 3 , Oblivion, etc run fine at very high details with no visible slowdowns. But the rest of them usually do this pls help

 

oh.. and i remember anothre really really weird thing that happened just when the problem started:
i was on xp sp3 , and when i installed a video driver it prompted me to restart , or the screen would be set to 640 480 4 bits of color.
before that , i didn't have to restart (and also, i tried it recently on xp sp2 and it did the same. and + it was sloooooooooooooooooooow ... the system overall was crawling. I have no idea what could be the problem.)
my bet would be the power supply , but i don't know and i can't afford to buy a new power supply right now

 

lol... dumb dumb dumb dumb dumb

i think installed them with antivirus on... lol lmao... me st00pid

well.. here goes ... trying to install again

 

nope... didn't solve it...

btw...
i remember that this started when i updated the bios.
i reverted to the old bios version, but my computer has been acting strange since i updated it. and it still is..
for example, before the update, when the system started, i pressed tab and the standard screen showed imediately. now i have to wait about 1sec for it to show. i really have no idea what the hell is wrong with my pc