When updating Exchange you probably con come across the following error in the prerequisite analysis:
A reboot from a previous installation is pending
The way to solve it is supposed to be by rebooting the server in order to apply any previously pending installed windows updates or whatever… but it never works, at least for me.
If it is also your case, try this:
Open regedit and access HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\
Apply. And retry the update installation. This time it should not return the pending installation error.