Don't think that's possible - certainly not in a header/footer.
You could use Worksheet protection to keep the casual user from
changing cell contents, but this is worthless to prevent the
moderately curious from gaining access. See
http://www.mcgimpsey.com/excel/removepwords.html
And there's really nothing stopping someone from copying your sheets
(without headers/footers) and pasting them to a new workbook.
XL is just not made for secure operations. Best bet is not to send
it to untrustworthy users.
MS is promising better protection through Digital Rights Management
in forthcoming versions - but that will only work for those who have
Windows Server 2003 and upgrade their versions of Office.
