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The Windows Installer REBOOT property can be set to "force", "suppress" and "reallysuppress". This last one raises some eyebrows- here is the break down:

Force

Always prompt for a reboot at the end of the installation. The UI always prompts the user with an option to reboot at the end. If there is no user interface the system automatically reboots at the end of the installation.

Suppress

Suppress prompts for a reboot at the end of the installation. The installer still prompts the user with an option to reboot during the installation whenever it encounters the ForceReboot action. If there is no user interface, the system automatically reboots at each

ForceReboot. Reboots at the end of the installation, for example caused by an attempt to install a file in use, are suppressed.

ReallySuppress

Suppress all reboots and reboot prompts initiated by ForceReboot during the installation. Suppress all reboots and reboot prompts at the end of the installation. Both the reboot prompt and the reboot itself are suppressed. For example, reboots at the end of the installation, caused by an attempt to install a file in use, are suppressed.

Another property, REBOOTPROMPT can be set to Suppress (or just S) so that any reboot performed by the Windows Installer happens automatically without interaction from the user. Setting this property does not initiate a reboot if one is not needed, it only suppresses the display of any prompts for reboots to the user.