Unable to view two excel sheets at the same time

Hi,

Ever since I had done an update (2 to 3 updates ago), i am not able to use Excel as soon as i open a second sheet. I can work with one sheet fine, then when the second sheet is opened they both minimize and when I try to maximize either of them it minimizes again. It is a nightmare to compare two sheets or move items from one sheet to the other. 

Thanks