Currently Being ModeratedNov 6, 2012 3:05 PM (in response to Bhikha)
You've installed a junk font that has the same internal name as the real Arial font, which it is bumping. Remove any recent third party font you added and the problem should correct itself.
Currently Being ModeratedNov 7, 2012 11:35 AM (in response to Bhikha)
I really didn't install any new font....
Maybe not directly, but an app you installed must have dumped more fonts onto your drive, as the one shown isn't any font that comes with OS X.
But how can I delete all the fonts I have installed myself?
Check the Fonts folder in your user account. If there are any fonts in it, remove them. Otherwise, compare the contents of the /Library/Fonts/ folder with this list and remove any which are not in the list.
Another way to find it is to open Font Book (or whatever font manager you're using) and click through them one at a time until the preview you see matches the bad font. Remove it.