This is more a *nix administrator question than a C++ question. Writing your own code to do this will likely break LAN security.
Assuming all PCs are Ubuntu (and you legitimately have admin level privileges):
Write a bash script that uses SSH to log into a PC and runs "sudo apt-get install vlc".
Normally, sudo must prompt for a password to escalate privilege but there's a work-around for that:
[SOLVED] Sudo in shell script[
^]
For an arbitrary, custom binary hosted on your own server, you can use SFTP to download it onto the client PC. You still need the SUDO work-around above to actually install it (eg. "sudo make install").