I just noticed a value of 0.000% for an annualized return computed using
XIRR in Excel 2003.
The probability of that result being correct is rather slim.

The formula used is =XIRR(B3:B56, A3:A56).
The problem appears to be caused by a Link.

If I instruct Excel to Open the Source for the link, the formula is
displayed as
=ATPVBAEN.XLA!XIRR(B3:B56, A3:A56)
If I open the workbook in Open Office's Calc, the formula is displayed as
=#NAME!XIRR(B3:B56; A3:A56)
If I remove the #NAME!, Calc seems to produce a plausible value.

There are over 100 XIRR functions in the workbook, thus far, this is the
only corrupt critter.

Looking back, I find a version of the workbook saved on 5 Jul 2008 did not
have this problem.
A workbook saved on 27 July 2008 does have the problem. I did not do any
Office Update in that interval.

How do I fix the Excel workbook?