I don't understand - it seems like this problem just started, but I've been
using the same file on both my desktop and laptop computer for several
months. It does contain macros.

Last time I opened it on my desktop it told me that it couldn't verify the
digital signature and that because I changed the file, it was removing the
digital signature upon saving.

Frustrated, I thought I'd have to re-sign it. I launched the MS Tools
"Create Digital Certificate" app that came with MS Office 2003. But it
clearly says in that window that it will only certify a file for use on a
single computer.

Then how have I been using this file without problem for a few months!?

How can I get my Excel file to work on both my laptop and my desktop, with
macros, without paying extra money to buy a 3rd party cert?

Thanks for any help,
Colin