pc lags, again.

I always keep upgrading this machine, but it still fails at some point, I also fully wiped hard drive and installed windows 7 few weeks ago, it worked ok (but not great) for thous few weeks and now it's same old story...
*PROBLEM
-random screen freeze for 1-4 sec when I'm typing, playing games (mostly League of Legends) or opening images, documents or audio files.
-screen becomes unclickable for few seconds when surfing internet (I'm using only Firefox)
-movies freeze for short period oftime (always about 8sec freeze 6-9 times in 90minutes movie, video files aren't damaged, it freezes randomly)
-recycle bin doesn't want to open, opens 6-10seconds after clicking on it and deleting 1MB files takes way too long (6-30seconds) same happened with win XP
-TOTAL FREEZE (while gaming only), screen freezes and you can see many red, green and blue dots on it, sound starts to buzz, keyboard and mouse don't respond at all.
*SYSTEM SPECS
Intel Core 2 Quad Q9450
GeForce 8800 GTX
ASUSTek P5G41-M LX Motherboard
Corsair dual ram 2gbx2gb=3.5gb
2 hard drives 250gb and 2000gb (2tb are split to 4 partitions, one is broken)
and XFX Pro 550W PSU
*Sensors from SIW
processor 40-55 C
video card 70-80 C
250GB HDD 40 C
2TB HDD 36 C
*Soft
Windows 7 Home Premium SP 1 32bit
ESET NOD32 Antivirus 6

I also ran Seatools from spunk.funk's signature or 250GB HDD (system hard drive) long and short tests passed.
Thanks.