Client is running MS Office 2010 on Win7 Home Premium, and the file association for xlsx is broken. It shows "unknown program". xls association (2003 era) is fine.
When trying to select an application, MS Excel is not offered. Browsing to other programs also does not show Excel.
Manually going to \Program Files(x86\Microsoft Office\Office14\Excel.exe does not work.
I have done a Repair Install of MS Office 2010 and this did not fix it.
I have also fully uninstalled and reinstalled Office 2010