Is this hibernation?

Squirrelnmoose was kind enough to suggest that this might be hibernation, but I didn't present all the symptoms in a previous post:

When my new Vista PC is left unattended for an hour or two it goes into the following state:

1) Blank dark screen
2) No response to clicking/moving mouse or tapping keys
3) Internet connection broken (no amber light on ADSL Link)
4) Surging hard drive

Switching the machine off by holding down its own power switch for 10 secs, then on again after half a minute, does not help.

I discovered that the only way to recover is to turn the machine off at the mains, wait for half a minute or so, and switch on again. Then everything is back to normal.

Is this called hibernation? It is more like a coma. It's the surging HD that is the most disconcerting.

If it is hibernation, what possible purpose can it serve? If it isn't, then is there something wrong with my PC, and what can I do about it?

Thanks for looking.