There is one more important distinction to note. Normally the command prompt, and therefore cmd files, don't support UNC paths.
However, using %~DP0 you can use relative paths to install software without mapping a drive. The command below is what I use to install Adobe Design Premium and it works from any path, UNC, Flash drive, or mapped drive.
msiexec /i "%~dp0\DP 32bit Basic\Build\DP 32bit Basic.msi" /q
I use this by typing \\server\share\software\adobe CS5 into the start menu or run command (xp). From there I can just double click a cmd file containing the command above.