Certain 3rd party software such as anti-virus and anti-spam have their own services running on the Exchange server and these can sometimes prevent Exchange services from being stopped successfully. This in turn can prevent the installer from updating files loaded by these processes and may require a reboot to complete the installation.
Since we have no control over 3rd party software, we’ve added the ability for you to register custom commands via a PowerShell script to stop and restart services, amongst other possibilities. Use of this mechanism may provide a way to avoid unnecessary reboots of the computer after update rollup installations. The script has three different sections – pre, post and rollback – that will allow the installer to take appropriate actions throughout the installation process.
Read more at source: http://msexchangeteam.com/archive/2010/06/02/455063.aspx