Launching Excel Attachments

I have a user that is receiving the following error message for every Excel attachment that they try to open from a message.

This file is not in a recognizable format

When they click the OK button, it opens two copies of excel. One version has the spreadsheet, the other contains garbage ASCII characters.

The user has forwarded the message to me, and I am able to open it, so it would appear that the problem is local. Does anyone have any suggestions?