RAM and/or hard drive problem?

In the last week, I've noticed my computer started acting odd.

I'm running Windows 7 Pro. Computer is custom build (built in Dec. 2010), Intel Core i5-760, G.Skill 8 Gig RAM, 4 Hard drives all Seagate Barracuda. At the moment, I can hear the hard drive constantly spinning as if it's constantly reading something. Performance is sluggish when trying to access some of the hard drives. For AV, I'm running McAfee.

However, when I check Task Manager, there's nothing running that would account for over 6 gigs of RAM being used. Is this just the ultimate fate of Windows 7 (which has never caused me any problems before, unlike prior Windows)? Or is there some other possible problem, I'm not seeing? Any help/advice would be greatly appreciated.

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I should also mention, this problem disappears temporarily if I reboot the computer. At some random point, usually a day or two later, this problem returns.

If you're sharing any files, someone else may be accessing the hard drive.

If not, does the same problem in "Diagnostic startup" mode?

Click the Start button, type in msconfig and press [Enter]. On the General tab, select "Diagnostic startup", click OK, let it reboot then test it for a while.

Once you're done testing, repeat the process but select "Normal start" instead.

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The Ultimate Boot CD (UBCD) has diagnostic utilities for testing hard drives and RAM.

If not done so already,
Take a look through the Resource Monitor.

So, for the past couple weeks, I've tried Shinma's recommendation and monitor the RAM through Resource Monitor, but I really can't tell what's going on. On a fresh restart, I have maybe 2 Gigs of RAM being used. It will stay like that for a day or so. As already shown in my previous two images, it seems Firefox takes up the largest chunk of memory while run, 500-650 MB on average. Still, if I were to guess at the total other processes it would only add up to about another 2 Gigs at most? I simply don't see in Resource Monitor or Task Manager what could possibly account for additional RAM.

It seems after a day of being on, RAM usage steadily climbs. By the time, it passes 6 GB, at least one hard drive starts spinning continuously. In fact, about a week ago, my machine froze up because all the RAM was taken up.

Any other suggestions before I run the diagnostic startup?

If you used the Resource Monitor,
It would have displayed the complete breakdown of Physical Memory in real time as well as corresponding service(s)...