Use the /norestart switch to prevent a message to restart the computer from appearing.

For example, I downloaded a hotfix from Microsoft with a filename of WindowsInstaller-KB893803-v2-x86.exe and saved it to my Desktop. So I ran the command:

“C:\Documents and Settings\username\Desktop\
WindowsInstaller-KB893803-v2-x86.exe” /norestart

NOTE: This works if you have version 5.3.24.3 of update.exe or later. For earlier versions, you can use the /z switch instead.