Font type is changing within Outlook 2003

When typing an email, a users font type is automatically changing after he hits enter at the end of the paragraph. It changes for the whole paragraph, and he doesn’t have any text selected. He is not using Microsoft Word as his email editor.
He has had similar issue before. The last time the issue was that his font size was changing and that issue seemed to resolve itself.
Any suggestions would be greatly appreciated.

The font type is changing from Arial 10 pt to Times New Roman 10 pt..