Hi! millwall.
Im not pretty sure about Excel been able to hide all sheets, but here's a
workaround if you dont mind displaying only one formated sheet.

1. In the Forms bar, click the Toogle Grid button so you wont the cells
border's, after that, select all the cells and fill them with some background
color you like. You can even rename the worksheet tab to " " (single space)
so it say nothing.
2. Open the Visual Basic Editor, you'll see a window called project and a
file called "ThisWorkbook", double clic on it, it will automatically open the
code editor, in the Object combo box, select Workbook, then in the procedure
combo box select the "Open" event. Excel automatically pastes some code like
this:

----------------------------------------------------------------------------------
Private Sub Workbook_Open()

End Sub
----------------------------------------------------------------------------------

3. Paste inside the sub this code:

----------------------------------------------------------------------------------
Sheet1.Visible = xlSheetHidden
UserForm1.Show
----------------------------------------------------------------------------------

4. It suposes you have two sheets, sheet1 is the one you want to hide, and
the other its the one you format. Hope it works for you.