While working in Excel, often times there's a need to use a calculator, here's a way to add one to your toolbar in Excel 2007.
First, right click on the toolbar and choose "Customize Quick Access Toolbar".
On the next Excel Options screen choose "Commands Not in the Ribbon" from the "Choose Commands From" drop down menu.
Scroll down to "Calculator" then click on "Add" and the calculator will show up on the right, click "OK".
It will now be added to your quick access toolbar. Just click it to open it.