My first guess is that calculation was set to manual.

I'd try:
tools|Options|calculation tab|check automatic

Excel picks this setting up from the first workbook you open in that session.

JAD wrote:

At work, I am developing spreadsheets using Excel 2003 on a desktop running
on the XP Professional. At home, I have loaded Office 2003 software,
including Excel 2003, on an HP desktop running Vista. Last night, I was able
to open and edit an Excel file developed originally at work and opened the
first time on the home Vista machine. When I got to work and opened the Excel
file, the first problem I came accross was a simple sum that was not adding
up. After looking at the most obvious reasons, I had a thought that it may be
the results of running and editting it last night on the Vista machine. Could
someone please let me know what the limitations are in this particuliar
setup? Your help is appreciated, JAD

--

Dave Peterson