MS Word 2007, Vista Home

I use Times New Roman as my standard typeface when creating documents. Yesterday, I noticed that when I open my documents, they now display in a sans-serif type. More oddly, the Fonts section of the Home menu tells me that the font I'm viewing is Times New Roman.

I have tried to reformat the documents, but every time I select a new font and then reselect Times New Roman, I return to the same sans serif type.

I have copied new Arial and Times New Roman files to the Windows fonts folder and have reinstalled Office with the Repair option.

How can I get my documents to display in the fonts they were originally created in?