User lost ability to open exisiting Excel files: what questions do I ask to help him?

Using Vista and Excel 2007.
One of my co-workers had a computer crash a few days ago.
Since then, he can open a new Excel file, but not an exisiting one.
Not one saved on his computer, not one on a network server - nothing.

As the most handy Excel monkey, I am trying to help him until IT gets around to calling back.
I usually do a shut down and reboot, and clear tmp files.

What else can y'all suggest?

Ed