I have an Excel workbook that was created with Excel 2000.  I has
extensive use of functions and formulas, some of which do not work
when I open the workbook using Excel 2002.  Specifically, WEEKNUM
which returns the week number of the year (week one starts January 1st
to week 52 ending December 31st) given a specific DD/MM/YYYY date.

How do I get these functions and formulas to work?  I will accept any
of the following solutions:

A link to an Excel update, plug in or patch that will get the function
to work or add the missing functions from Excel 2000 back into Excel
2002.

A work around using functions available in Excel 2002

Some other solution that gets the functions and formulas to work!

WEEKNUM is available in excel '02, but you need to install the
Analysis Toolpak.  From the '02 help:

WEEKNUM

Returns a number that indicates where the week falls numerically
within a year.

If this function is not available, run the Setup program to install
the Analysis ToolPak. After you install the Analysis ToolPak, you must
enable it by using the Add-Ins command on the Tools menu.