Thanks for your input so far. Let me tighten the scenario a bit to make it easier to see where I'm going:

1. No hardware malfuncion.
2. System not virus, spy- or malware infected.
3. No messages in the event log.
4. System fully updated.
5. No recent change in hardware or software.
6. No network activity.

Just to make things clear, I'm not experiencing any trouble atm. I just want to know your best tricks for next time it happens. Because it will happen. It always does sooner or later.

About hardware malfunction: I've experienced both faulty memory and low quality PSUs. And it's really hard to find those problems since they can result in just about anything. And buying cheap PSUs are probably the number one sin of all home built PCs. 

But let's leave it out of the equation and make it a windows only problem. I'm making this scenario hard since most of the easy questions can be answered by googling or the knowledgebase. I want to know how to go "under the hood" of the system and pinpoint the exact component.

/Lars.