password protecting sheet when workbook opens

I want a sheet in my workbook to be automatically protected when it opens. So, I put this into 'ThisWorkbook':

Private Sub Workbook_Open()
Password = "jack"
ThisWorkbook.Sheets("invoice list").Protect Password:=Password
End Sub

Then I closed the workbook, reopened it, and the sheet was protected. However, I am now unable to unprotect it, as I keep getting told that I've got the wrong password!

Firstly, does anyone know how I can unprotect this? Secondly, what did I do wrong? how can I get this to work?

Thanks!!

ok, I managed to unprotect it using this rather impressive tool:
http://www.pcworld.com/downloads/fil...d,22990,00.asp

But, I still don't understand why this happened. Any ideas?

Problem solved!

This seems to work, basically specifying the workbook. I don't understand the difference - I guess it's just one of those little mysteries....

Private Sub Workbook_Open()
Dim Password
Password = "jack"
Workbooks("INVOICESv2a.XLS").Sheets("invoice list").Protect Password:=Password
End Sub