Does it only happen with your email program or other word programs too? and what email program are you using?
Since when is this happening? You could try a system restore to an earlier point you remember it working right following this tutorial: System Restore
Or download and install the Arial fonts from here: Search: Arial | *Font Download - The World's Biggest Font Site
Let us know if the problem still persists.
Edit, download and unzip this file. It contains all the Arial types.