Hi Zoobadabadoo,
I had exactly the same issue as you, all my Arial text files, and YouToob etc...
not displaying correctly with a weird graph-style font.
I opened a textfile with the corrupted Arial font, then opened Fontbook
and browsed until I found the same font that replaced the Arial font....
It is the "jungle" font, displayed in the list of Fontbook-app, all you have to do
is command-click this font and choose "remove font"= system instantly puts the correct arial font back.
Hope this helps
Biggnik