If your running Windows 7 try this
http://support.microsoft.com/kb/2257542
The cause seems to be:
The problem of not being able to see Excel files, while the File / Open dialog in Excel is a known issue and our engineering teams are still searching for resources. The problem stems from the fact that Windows 7 file dialog calls back to Excel to generate the preview, but Excel is waiting for the user closes the dialog box before it can respond to the request for Windows.
Checking through, the first link may help, but it seems to be a known problem, which MS are "working on" ;-?