I found this tip here and tried it in PowerPoint. It seems to work. After typing the ` I switched back to Arial and got the c with the overbar.
Before you type the character for which you
want the overbar, change the font to Symbol.
To create the bar, type the ` character (accent
grave, may be above the Tab key)
Then, stay in Symbol font, or switch to a different
font, and type the character that has the overbar.
2 - My corporate image blocks this site so I can't look at it right now, but maybe it will help: