Every once in a while, a hidden POWERPNT.EXE consumes 100% of one CPU core (on a single core, that is deadly, on a multi-core system it drains your battery pretty fast).
This was the cause:
One reason is you have preview pane enabled and have selected a PowerPoint File. To preview it an invisible copy of PowerPoint is opened and may not close when you deselect. This doesn’t normally use much cpu though.
–jeroen
via PowerPoint high cpu usage.
The Wiert Corner - irregular stream of stuff 