Windows 7: File opening

Every few weeks my Windows 7 (Home Premium) takes it into its head to start opening files and folders half-size, so I have to click top right between 'minimise' and 'close' to bring them up to normal full screen to work on them.
I have lots of folders within folders so it becomes time-consuming to have to click top right every time.
During say half an hours work they stop doing this i.e. they open full-size but it might start happening again the next day.
I know that this half-size opening happens after I have clicked on 'set as desk top background' but why does it happen at other times?
Weeks can go by when it is ok.
Can I do anything about this?

Some alternatives.

F11 to make the window real FUll Screen.
Windows key + Up arrow to maximize a window and Windows key + down arrow to restore to previous size.
Windows key + Left arrow maximize to left side of screen.
Windows key + Right arrow maximize to right side of screen.