If your computer has been slowing down periodically recently it may have been down to a Microsoft fuckup in a recent windows update. The routine update file within an update package once downloaded could not install because it was too large for the destination drive partition. This meant it was left hanging and repeatedly attempted to install periodically taking up large amounts of available memory to do so.

3 weeks down the line MS have finally started rolling out a fix (you don't need to do anything)