What can be the reason for system freezes that are so "hard" that even the hardware reset button takes about 3 seconds until it actually resets the system (and then it actually powers down and up again instead of doing a "clean" hard reset like when pressing it during a normally running system).

Since it initially happened mainly while playing videos from YouTube I suspected the graphics card - however, I replaced it recently and it did not change it. It still happens from time to time (and sometimes more often, like a few times times in the last few hours).

The system is running Windows 7 - but I don't think this matters since I don't think any software, not even the OS, can actually affect the reset button's behaviour.

The PC is not overheated and the freezes happen randomly. There is also no malware on the system. The CPU is an Intel Core i7-920 on a Gigabyte EX58-UD5 mainboard.

What could be the cause for this problem? Faulty RAM? I did not run a full memtest86 check yet, but I wonder if there is a more likely issue than faulty RAM - checking 12G of ram does take some time after all!

There are no entries in the event log - but that's what I expected since the system freezes so hard that I doubt it has time to write anything to any log.