Usually a sign of bad memory. The BIOS memory test is not worth the time it takes to run.
True. I recommend: http://www.memtest86.com/
I think we're fortunate regarding the quality of power distribution here in Sweden. It's very reliable. And reboots due to power loss are rare in my experience. It's more common that people buy the cheapest PSU they can get, and overload it with power hungry graphics cards and overclocking. But then they're just asking for trouble.
In my scenario the computer isn't broken. There is just something that makes the GUI (explorer.exe) go to sleep. It is a wait that is not a busy wait. All the vital signs (processor/disk/net/mem activity, cpu temp etc) say that the system is idle. The task manager and the mouse works as usual. And it's possible to launch new programs from the task manager's run. As long as these don't open any explorer or IE windows. But the normal explorer GUI does not respond. Explorer is waiting for something that makes it unresposive. I'm sure we've all experienced this one time or another. The question is how to pinpoint what it is that is causing the GUI (explorer or IE) to go to sleep in each instance.
I was hoping that there was some good utility that I never heard of that would let me peek under the hood of the explorer and see what is it doing. Dependency walker looks promising tho.