Hi Guillaume,
I'm working on a GPS software and discovered somewhere (don't remember the link) an excel spreedsheet that have conversion from WGS84 to GAUSS-BOAGA (which is what I was searching for) and calculate the distance (and direction) between to coordinates, which is what you need.
The file is not mine, and I have not tested it yet, so do some test by yourself....
The file is also in Italian, so translate it!!
You have to look at the sheet named "CALCOLO DISTANZA" (in english "calculate distance"), I have uploaded it to my server:

http://www.cfsolutions.it/Conversione_WGS84-GAUSSBOAGA_CALCOLO_DISTANZE_E_DIREZIONE.zip

please let me know if it work correctly!!

HTH

franco

>Hello,
>
>Does anyone know how to calculate the distance between 2 coordinates ?
>
>More details :
>I have the coordinates (longitude + lattitude) of customer X and I have the coordinates of customer Y.
>I have to calculate the distance between the coordinates (bird's-eye view or how do you say that :o)
>
>Any ideas ?