Windows 7: Slow PC, Hang Ups..Desperation

Hi. First post here and I think it's a very nice forum.
I have a problem for at least a week now. Once upon a time my pc was very fast. Recently though something happened. My pc hangs for a few seconds (or even minute) not responding to any new actions.The mouse is moving though and when the hang is over, BOOM, all actions take place as if they were queued. At this time, my HDD is working at it's fullest! The hang ups happen mostly random but more often when the HDD is being accessed at a frequen basis. Even when hang ups not happen the whole system is like a pendium 2

SPECS
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ASUS P6T
i7 920
2x3 Corsair RAM
1 WD HDD 1TB
GTS 250
Windows 7 7600

What i did:
-extensively scanned my HDD with Data Lifeguard but no errors at all!!
-scanned for virus, spyware etc etc. It's all clean now (I had one but its irrelevant).
-disabled indexing
-disabled superfetch
-change power saving options
-installed intel matrix storage management
-installed latest drivers from ASUS website
-changed HDD cables
-changes connectors
-load bios defaults
-changed from IDE to ACHI and back

The problem was little in the start then one day i unziped a BIG file at which time a BIG hang happend.After that time hang ups are far more frequent, boot up takes far longer as well as even shut down!

I can only believe its my HDD but all test I've done on it tell me it's FINE..

Thanks in advance

-edit
The exact same happens even in Safemode... or with every process closed

I would try running safe mode to confirm the system doesnt hang if so then i would reboot enter msconfig, startup tab and uncheck everything,then services tab check hide all ms services and disable whats left then reboot and see how you go if its ok then you know its a third party app and you can reinstate them one by one till you find the culprit if it doesnt help post back

Hi,
As I've told in my post edit. The same problem happens in safemode too unfortunately..

Ok, think i was typing while you edited
Have you tried leaving task manager on the screen while its hanging to see where all the cpu/ram usage is going, may give you an idea.
Also try opening cmd.exe right click run as admin then type sfc /scannow
to run a system file check

Hi again..
Yes I've tried task manager and even Moo0 SystemMonitor
Task manager doesn't show anything that is using high resources and Moo0 SystemMonitor just show full activity of the HDD (100%) at the time of the hang up. As a concequence it indicated the the pc is bottlenecked by the HDD.

I try reinstalling windows on top of my installation..Despite the fact that it did about 2 hours, now it is just stuck whenever trying to load services in the setup process. I've tryed "setup rollback" but it reboots my pc...
I mean...this pc is doomed

Ok...an update for those who have the same issues and they are many as I can tell.
I saw by the windows monitor that my HDD highest active time at the hang ups, was 100%
So I googled around a bit and found that some people had those exact same issues with Western Digital Green
That happens to be the HDD I have too.
Some suggested to use WDIDLE3 to reset the idle time. (though I have no idea what it is and how it's done.iLL find out)

(I can't be so unlucky. First I had the seagate barracuda which had an issue with firmware and now WD green.FFS!)