All Physical memory being used but nothing showing in task manager

i had the same problem and just resolved it.

for the past month my computer was running sluggish and its memory was at either 77% or 93%, even if nothing was running. in task manager all the different processes added up to about 1.75 gigs, so there was about 2 gigs of memory unexplained for (4gig memory). i always ignored the "show processes from all users" because im the only user on my computer, but i decided to click it finally, and a new process popped up on the top of the list. Window Media Player Network Sharing Service, this process by itself was using 2 gigs of memory. after ending the process, my memory returned to normal. however another thing you have to do is go to the resource monitor in the performance section of the task manager and suspend all the Window Media Player Network Sharing Service processes that are still open slowing things down, and then your pretty much set.

i realize that this might not be the exact answer to everyones problem but give it a shot.

This problem plagued me for much longer, and required much more effort to resolve, than it should have.  I have been running an 8 core, 12 GB Dell that would go from 4ish GB to 11.5GB in the matter of 10 minutes, without any warning.  Google searches led to a plethora of failed attempts including shutting down the Windows Media Player service, uninstalling VMWare, and removing AVG anti-virus software.  The problem didn't budge.  One aspect of the problem that I noticed was that I was using much more RAM on start-up, and overall hard-drive space, than I anticipated.

I believe your culprit is Window 7 itself:  the Windows Search service. From what I can glean, if you do a lot of computationally heavy work which creates a lot of temporary files, or have a large number of files (not large files per se, but a large number of files) on your hard drive(s), when this service kicks in, it attempts to index them, and store the resulting index in memory for quick searching.  This leads to two problems:

(i) a huge Windows.edb (basically, the physical file representing the index) 

(ii) a massive amount of used memory (which doesn't show on task manager), when this file (or aspects of it) is loaded into memory

A nice tool, which in fact on a MS hosted site and should be much more accessible, to see your ACTUAL RAM usage is RAMMap http://technet.microsoft.com/en-us/sysinternals/ff700229.  This will show you the offending culprit.

Blowing away the service and the Windows.edb file not only solved the problem of interest, but added 70GB to my hard drive, and lowered my startup RAM usage to 2.5GB (from 4.5GB)!  Here is a good starting point:

http://www.howtogeek.com/howto/10246/how-to-disable-search-in-windows-7/

For the record, I find it to be bad OS development to allow a system file to get in the realm of hundred of MBs, and really awful development to get into GBs.  A 70GB (almost 12% of the total hard drive size) OS file speaks volumes for Windows 7 development.