suffice to say that, when in windows, you can't get the task manager open (and deal with whatever problem from there), it's time for a hard reset. You may be able to slide the cursor back and forth 'til the cows come home, yet clicking on anything yields no response.
This is what I mean by "The computer has stopped responding".
On very rare occasions (and usually in linux or os x), a usb keyboard (and anything daisy-chained to it) will stop working until I unplug and re-plug it.
I've also seen hdd lights continue intermittent blinking for long periods after a pc has hard-locked. I wouldn't say the lights are completely useless, but by the same token they aren't completely reliable, and certainly not essential.