datevalue function under excel XP

I hope someone here can help me out. I use a spreadsheet
function to look at a column that contains dates in the
mm/dd/yy format. I used the conditional sum wizard under
excel 2000 to build a formula that will look at these
date values, find all that are within a month period, and
add a value in another column if the date was within the
specified parameters.

Here's the formula; column F contains date values, and
column K contains the value to be added.

=SUM(IF($F$2:$F$149>=DATEVALUE("12/1/2003"),IF
($F$2:$F$149<=DATEVALUE("12/31/2003"),$K$2:$K$149,0),0))

This formula was built under Office 2K Professional.
When I opened the file to update it, running XP Pro and
Office XP Pro SP1, I cannot recreate the fomula for
subsequent months. I tried simply subtituting the other
datevalue targets, but it returns a value of zero, when
there are clearly dates within the column that fall
within the range or 1/1-1/31/2004. Manipulating the date
format in the formula makes no difference.

So I copy the file over to my laptop - still running
office 2000, and the formula works as expected. Worse
than that, if I save it there, and copy it back, the
formula works as expected. I have adobe acrobat macros
installed - no other add-ins except the conditional sum
wizard.

What, if anything, has changed with regard to the
datevalue operator? Is there something wrong with the
way that I am approaching the problem?

Any and all help is greatly appreciated.