I've installed the Analysis Toolpack addin and have used the =weeknum()
function in the past. However, this morning when I openned up the
spreadsheet I use it on, I get the #NAME error in that field. This has
happened to me once before, but I forget exactly what I did that solved the
problem. I've tried saving the spreadsheet and re-openning it, as well as
using the function in a brand new workbook.

I have Windows XP Pro w/ SP2 and Excel 2003.