@Maziar : I didn't overclock the niether the CPU nor the GPU and I will try re-installing windows but I want this to be my last choice.
@icraig : It mostly crash when I'm working on it. I have a Gigabyte GT 240 , I don't think it's a VGA drivers problem because it's written that it supports windows 7.
did the crashing start after you installed your GT240? when did you install it?
also did you install any software, driver, or any new hardware?
also, download this app: http://downloads.guru3d.com/HWiNFO32-version-3.56-download-1078.html
install it and go to the monitor tab in the software.
then start this app: http://mersenneforum.org/gimps/p95v2511.zip
and start taxing your CPU.
after 15 minutes, take a screenshot of both the application and post it here. if you PC crashes before you can take a screenshot, run it for a shorter time and take the screenshot