Making macros universally available (various versions of Excel)

I'm working on a macro to help our sales staff format some of their reports. They're being sent .TXT files and need to convert some of the formatting, so the macro takes certain columns and formats them as text/numeric/date/currency/blahblahblah.

What I'd like to do, though, is make the macro available to them without their having a particular file open--i.e. so they can just open up Excel, go to the Macros menu, and select it regardless of the file they have open. Basically to tie the macro to Excel rather than to a specific file.

I read about Office 2007 having a "personal workbook" feature that allows for this sort of thing, but since some of the people this will be servicing are at remote locations and acquire their own PCs, we have to work with versions potentially as old as Office 97. I've been told we won't bend over backwards to support 97/2000 users and will primarily stick with XP/2003/2007, but if there's an easy way to make it work with back versions, that would be great too.

Thanks a million. I don't know what I'd do without you, but I suspect it would involve clenched-fist-induced hair loss.