Screen suddenly goes off on Vista computer

That's 1 year since I got a PC-based Core 2 Duo E6750 with 2 GB of RAM. I have chosen quality components (Gigabyte motherboard, G. Skill memory, Seasonic power supply, Zalman cooling etc ...)

I'm on Vista SP1 32-bit. Everything works very well for almost 1 year. But since this summer it is that I see a very strange fault: I work on the PC, and a sudden the screen goes off, and go to standby! The PC will continue to work (hard drive activity, playing a mp3 that continues ...) against the network by no longer works, the machine is unavailable (either via VNC or ping, and it has more 'network activity). It is impossible to do anything and suddenly only a hard reset is required, making me lose my current session.

When I restarted, there is no problem, Vista just said to me that the PC was not properly booted, but no trace of any problem in the logs. The worst part about it is that the problem occurs at any time, totally random, no matter what I do on the PC. I listen to music or not, I plugged this device or not. Often while simply surfing the net when I'm done that. Sometimes this will occur until 4 or 5 times a day, sometimes it will not for a whole day. Sometimes it will do 15 minutes after restart, sometimes it will be 4 hours later or not at all. I just noted that he had never done when I leave the PC turned on but I do not use it (it is often turned on a whole afternoon without my key, and there by chance the problem does not appear and the screen saver with photos running smoothly when I'm back).

I said that I have a fanless MSI graphics card based on GeForce 5200, I am not gaming, the temperatures are correct, the voltage. I also tried plugging my monitor on the 2nd DVI port behind the card at the time of the problem, but obviously it did nothing. I tried to change some options in the BIOS, but nothing happens, unplug USB devices that I do not at times but I always had this "failure" even with some peripherals disconnected. I have no other PCI Express graphics card on hand to see if it comes from, but it would be really curious for almost 1 year everything has worked very well!

Anyway I have tried everything. I do not know what to do. If someone has an idea or has been facing the same problem!

Do you have inflated components and/or blackened to your motherboard and your GPU? Your PC is not dusty? Your PC is it overclocked? You getting a Windows install from? Have you already done a memtest and OCCT?

No overclocking, I even tried to turn disabling all functions of overclock that are now automatic in the BIOS of motherboards. The PC is well maintained and not dusty, I have a glass on the case and so I take care of the interior.

I do not reinstall Windows because that is precisely what I'd like to avoid, I will lose lots of time to start over ... backups with things of all kinds, updates etc ...

For tests, MemTest and OCCT, no I did not do that and I intend to try this tomorrow, actually, it costs nothing to try it! I want juice!

Otherwise, what the temperature "System temp? + It is warmer than the CPU in fact it is only very rarely 40° while the system temp is frequently between 50 and 60°. I do not know if it is serious?

Would this be a SB/NB that heats? This is only a supposition.

May be the radiator in the chipset, for some reason, is poorly placed would probably cause such crashes?