If you have a drive in PIO mode, you get very high CPU usage by DPC process in ProcessExplorer. My unlucky desktop had something else that occasionally triggered DPC choke, but it disappeared after reboot. I don't know what that is except that it has something to do with my MB. Just be sure you are running with admin rights.

It may very well be something with your hardware, like chipset/CPU problem. Oh, just remembered - some cases of "slow computer" were caused by broken cooling/dust and CPU engaging thermal throttling. I know Everest can check that (Tools -> System Stability Test graphs).
[OT]Can anyone point me to other (small/easily portable) open/free program that can do that? [/OT]

And have in mind, that if you download updates for Windows Defender, it silently scans your hard drive every month.
Or have you lately installed some new indexig software?

I had a wireless keyboard (2xAA!!!) (and used a mouse on a few ocasions).

Wireless means you suddenly stop moving while you see a rocket going your way. Or get behaviour like "thisttttttttttttttttt" typing in commands in terminal. And battery indicator still shows 30% power.

Wireless means you find yourself wondering "where the hell is my keyboard" after watching a movie from your bed last night and moving the bed around to find it behind in dirt. The mouse gets around even more (as it's smaller).

My keyboard was Logitech something, batteries lasted for about a year.

Wireless is not Nirvana, you know?

http://en.wikipedia.org/wiki/Colobot You can do most (if not all) tasks by manipulating robots yourself