Well, having started as a recent issue, this generally points to a virus or physical hardware issue. Could still be a memory leak flaw from Windows, some update that was installed that caused more harm than good. They HAVE been known for that in the past. Run Windows Update again and see if there are any new ones for your system. They usually detect flawed updates and correct them rather quickly.
Running MalwareBytes and AVast are great and all, but the problem with free AV software is that it is generally lacking crucial up to date information. Also, even Norton won't always find and remove a rootkit. To remove a rootkit, one has to go low level, before Windows has even started to boot. McAfee has a free one with instructions here:
http://www.mcafee.com/us/downloads/free-tools/how-to-use-rootkitremover.aspx
MalwareBytes has one as well, but it's in beta stages.
https://www.malwarebytes.org/antirootkit/
Norton's:
https://support.norton.com/sp/en/us/home/current/solutions/kb20100824120155EN_EndUserProfile_en_us
Microsoft's virtually useless advice:
http://www.microsoft.com/security/portal/mmpc/threat/rootkits.aspx
And my personal favorite, Kapersky:
http://support.kaspersky.com/viruses/solutions/5353
Rootkit detection and removal is a long and boring process but needs to be done every once in a while since protection against them is more difficult than for standard and commonly encountered malware.
For memory leak issues related to Windows, check what your Paging File size is set to. Normally, it should be set for letting Windows determine the size. If it isn't set for that and has a specific Page size, then you might want to try increasing the available size. Windows uses Page as a "virtual RAM" for files it is accessing a lot, but doesn't need in memory all the time, which also won't show up in the Task Manager. You could also look for program errors in the Event Viewer to see if the RAM use is due to a memory leak caused by another program not letting go even after shutdown. Simple way to access both areas with decent instructions: Window icon, type in the Search box Event Viewer or Virtual Memory or search either term from Window's Help system. The Help system has come a long way since it's introduction back in 3.0.
As to a possible physical RAM problem, download and run MEMTest which will stress your RAM and check for flaws. If it comes back fine, you may also try swapping RAM sticks out with another set and see if the problem persists. Not everyone has spare RAM laying around, so I can understand if this is not really an option, but if you have a friend with the same type of RAM as your own, you might be able to borrow theirs for a quick test.