I'd create a named range and then just clear the cells in that named range.

Select your range of cells that should be cleared.
Insert|Name|Define|
and give it a nice name (I'll call it Input).

Then create a macro like this:

option Explicit
sub ClearInput()
 worksheets("sheet1").range("Input").value = 0
end sub

The plop a button from the Forms toolbar and assign that macro to the button.

In fact, I think I'd verify the request just in case:

option Explicit
sub ClearInput()
 Dim resp as long
 resp = msgbox(Prompt:="Are you sure you want to reset to defaults?", _
                buttons:=vbyesno)
 if resp = vbno then
    exit sub
 end if
 worksheets("sheet1").range("Input").value = 0
end sub

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm