If you are using Office Excel 2007 click on the Microsoft symbol on the left top corner then click on excel options then check the check box which says Show deleveloper tab. Now you can see a menu "Develepor" added in excel click on it and you can go into the desing mode to build the VB form and then VB macros to write your functional logic.
If you are using Office Excel 2000 this option is already enabled in the View->Macro menu. Hope this helps.
I don't build Software but I make them work better, Testing is a Passion