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