keyboard shortcut for paste in Powerpoint not command V
I had the same problem in Excel pop-up out of nowhere. Suddenly Cmd-V wouldn't paste anymore. Checking in the "Edit" menu I found that Cmd-V was associated with "Copy to Scrapbook", while Cmd-X would paste, etc. a whole cascade of messed-up keyboard shortcuts (of which only the lack of Cmd-V for "Paste" bothered me).

The following worked within Excel:

Go to "Tools" > "Customize Keyboard..." and reassign Cmd-V to "Paste" by selecting "Edit" under Categories and then "Paste" on the right. "Cmd-V" was still among the 4 or so "current keys" there but reassigning it still solved the problem within Excel.

For PowerPoint there is no such functionality it seems, the Microsoft website refers to "System Preferences" > "Keyboard" > "Keyboard Shortcuts" > "Application Shortcuts" where one can specify shortcuts for PowerPoint. 

This obviously doesn't fundamentally solve the problem of how the keys got reassigned.

And I just realize that my answer doesn't directly answer in your situation. Nevertheless I hope it helps somebody else with the problem in Excel or Word.