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.