I have Windows 7 Home Premium with a AMD X4 965 Quad Core and 8GB of Crucial RAM. I have no idea why my computer says it is using 6GB of RAM when it doesn't show any reason. Below are the screen shots of my task manager:
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Caching. Your RAM is tens of times faster than your HDD, so starting with Vista, windows aggressively caches program data. It's a good thing - RAM'd be useless to you otherwise.
(It frees up if it's needed, don't worry)
edit: BlueRaja makes a good point - anybody interested should definately read Jeff's Post on the subject - his insights are once again invaluable to us all :)
As long as you are not near 95%/full, I wouldn't be that worried as Windows is very good at managing memory.
However, the usual cause for such high usage (as it does seem a little high to me), is a memory leak in a device driver.
It is really hard to tell from those images as they are small, but it looks to me like you haven't checked the box in the bottom left hand corner of task manager
Show processes from all users.
On top of this, I recommend you use Microsoft / Sysinternals Process Explorer as it is a much enhanced version of task manager that allows you to delve deeper in to what is taking up memory.