Unfotunately the buttons on an Excel sheet are contained as part of the Excel file. So minimizing Excel will take the button with it. I'm not aware of any way to fix that.
You may be able to get help doing this using VB.Net at http://www.xtremevbtalk.com/ though... Not promissing, but they may be able to find a way. The solution won't be driven from Excel though, that I can assure.