PC Freeze&Crash at several Games

What I Do : I just start CoD4/CoDWaW/CSCZ and start playing, PUBs or PCWs, doesn't matter. Suddenly, my screen totally freezes and my sound keeps repeating the same sound over and over again (loop) -> My PC totally doesn't respond anymore. All I can do is Reboot my PC, but after 5minutes, I could have the same problem again; although, I don't have the problem every day^^ Still, this is VERY annoying. Also, I only have the problem on Multiplayer~>NOT on Singleplayer or Co-Op. On Singleplayer, I get a lot of BSOD's, but no Freezes.

I tried a lot of things till now, from other forums & help guides, but nothing have helped yet. My PC can handle COD4 etc easily. I have 333FPS steady. I do have some doubts about my GPU Temps....
For CPU 30->55°C
For GPU 45->69°C

~> Most people say these temps are fine, but my FAN Speed goes up to 100% pretty quickly and GPU Temp to 69°C when I'm like playing 1 hour. (And I play like 15 hours each day so...)

- PC stats :
Intel(R) Core(TM)2 Quad CPU Q9550 @ 2.83GHz (4 CPUs), ~2.8GHz
Kingston Valueram 4094MB 1333MHz DDR3 CL9 DIMM KIT
Western Digital 500GB Caviar GreenPower SATA2 @16MB
ASUS nVidia GeForce EN-GTX260 896MB DDR3
Plug en Play-monitor Samsung 245B+ 24"
SoundMAX Integrated Digital HD Audio
Creative Headset Fatal1ty Gaming
Creative Inspire A500 5.1 Surround Speakerset
Logitech G15-Keyboard Gaming Board
Logitech MX1000 Wireless Gaming Mouse
Windows Vista™ Home Premium (6.0, Build 6001) Service Pack 1 (6001.vistasp1_gdr.080917-1612) 64-bit
ASUS P5Q3 Deluxe WiFi 775 P45/ICH10R DDR3
Antec PSU TruePower Quattro 850W 24P ATX 12v fan 80mm
Gigabyte GZ-AA3CB Poseidon 310 Black

- Other Programs I have running with my games
* Xfire (usefull, so rather have this on)
* TeamSpeak/Ventrilo (realy need this)
* AVG Anti-Virus (rather have this on)
* Windows SideBar
* Sometimes Windows Media Player (entertaining @ PUB)
* Sometimes FireFox or Internet Explorer
* Sometimes CoreTemp
* Sometimes GPU-Z
* Sometimes CPU-Z
* Etc....

I have already tried to play without these programs on, but nothing seems to help... Also I did a MemTest86, but it hasn't find any errors. I do have the latest drivers for almost any driver-needed-thing on my PC. And I do have a lot of coolers in my PC ! My guess goes to a broken GTX260 Card :s ?

I would like to do Officials as fast as possible but as long as my PC does this weird.. I cant. If any1 has any solutions or questions about this, go ahead plz!