Hi Andrew,
Sometimes, Windows Installer can determine when a reboot of the system is necessary and automatically prompt the user to reboot at the end of the installation even if no reboot is scheduled by the author of the package.
For example, the installer automatically prompts for a reboot if it needs to replace any files that are in use during the installation.
For more information please visit:
http://msdn.microsoft.com/library/defau ... eboots.asp
All the best,
Ciprian