The source code and jar files are available here. The code is distributed under GPL license. See the license file in the archive for more details.
To unpack the archive enter:
tar zxvf geoconv.tgzUnder windows, I would recommend that you format your hard drive and install a decent version of linux instead. If that is not an option, winzip shareware should do it.
To execute the program you need to install a java virtual machine on your system. See SUN java page for more details. Then just enter:
java -jar geoconv.jarRun "java -jar geoconv.jar -help" or read the README file for some usage guidelines.
I found all informations I needed to implement these conversion routines on the IGN web site (French National Geographic Institute). In particular you will find general information on geographic coordinate systems, information on French projective systems, and coordinate conversion algorithms.
The Swiss coordinate conversion is documented here.
Eric Sibert has also been developping a coordinates conversion software, available from here, now in open source.