Finally I've formatted the system and reinstalled Windows 7, with all the latest driver I could find. Unfortunately, despite idle activity, the system still continues to crash! This time without any programs running except for Windows Explorer.
The screen simply went black and the system is completely unresponsive. Hard reset as usual.
I'm strongly convinced that the GPU might be malfunctioning, but since I could run 3D games and other graphic-based applications without any problems, I'm still highly doubtful.
I've decided to check on other forums regarding the Forceware 258.96 graphics driver and found loads of other similar cases, including crashes and instabilities, despite idle or strenuous activity.
Finally, I managed to retrieve the original graphics driver that I've installed the first time I installed Windows, the Forceware 197.16. Though gaming performance is slightly lower than 258.96, I'm pretty sure that this driver is a lot more stable...well, at least if my GPU is still intact.