Grid Check problems

Ben

Whenever I check the grid, the most common message I get, and the one I can't seem to fix, is the error that states:

Internal angle exceeds prescribed value

or

Internal face angle exceeds prescribed value

The mesh I have a square in a circle, just like the ones in the help books that are examples given for surfaces to be extruded.

How do I fix this problem? As for the future, how do I avoid it?

Thanks