One method that would be a little more secure, but at the price of being less
convenient for you to view/work with this sheet would be to:
Open up the VB Editor ([Alt]+[F11])
If the Project - VBA Project window is not visible (says that at the top of
it and has list of worksheets in the workbook in it), choose View [Project
Browser] from the VB Editor menu (or [Ctrl]+R)
Click on the worksheet name you wish to seriously hide. Look at it's
properties in the Properties window. As with the Project window, if it isn't
visible, choose View | Properties Window (or [F4])
at the bottom of the list will be the Visible setting. Use the dropdown
list to set it to xlVeryHidden.
This makes the sheet invisible and keeps it from showing up in the list of
sheets available to be made visible while in Excel.
This alone may be enough (combined with workbook password) to keep prying
eyes away from it. But if you want even more protection:
Decide on a VB Project password. Write it down somewhere - if you lose
this, it will be very difficult to get back into your VB project to make the
sheet visible later. While still in the VB Editor and with your sheet set to
xlVeryHidden, go to the menu bar's Tools option and choose VBAProject
Properties. Click on the [Protection] tab, check the "Lock Project from
Viewing" option and enter and confirm your password.
Close the workbook and reopen it and you should not see your sheet, and you
should not be able to get into the VB Editor without providing the password.
To work with the sheet, provide your password to get back into the VB Editor
and set the sheet's visible property back to xlVisible.
I think that's going to be about as good as it gets, maybe. But as Gord has
noted, bypassing all of this kind of thing in Excel is rather simple. I
could take this type of protection, open another workbook while your's is
open and write code to make all sheets in your workbook visible to me. A few
minutes with any of several Excel password cracking programs and it's all
available to me.
Gord's final advice of "if you don't want someone to see it, don't make it
part of the workbook" is the only sure way of keeping it confidential.