How to prevent new workbook opening if other workbook already open

I have Excel 2007. I am using the excel button from my Advent/AXYS program. I run several reports and do not want a new book to open each time. I read it may be a problem with the extension handler so I went to My Computer > Tools > Folder Options > File Types > selected ".xls" > Advanced > selected "open" and tried to change the default, but when I click on "open" I can not choose default as it is grayed out. I am not even sure if that is how to fix it. Is there a way to stop excel from opening a new workbook? Thank you.

Perhaps starting excel using the "e" switch? See link

http://office.microsoft.com/en-us/ex...in=HP010197489

Alf

Thank you for the suggestion, but this did not fix the problem.