MS Excel 2010, Run time error 91

Every time one of my users open MS Excel on his computer, he recieves an error message saying:

Microsoft Visual Basic

Run-time error '91':

Object variable or With block variable not set


When he clicks End, the warnign closes and his file opens as normal, however it also opens a second instance of MS Excel but that instance is just a blank spreadsheet.

Any ideas on how to fix this?

Thanks
Marth