Perhaps this will help you:
To prevent Automatic Updates from restarting a computer while users are logged on, the administrator can create the NoAutoRebootWithLoggedOnUsers registry value in HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU. The value is a DWORD and must be either 0 (false) or 1 (true). If this value is changed while the computer is in a restart pending state, it will not take effect until the next time an update requires a restart.
regedit -> navigate to (create keys if necessary) HKLM\software\policies\microsoft\windows\windowsUpdate\au -> create a new DWord value named NoAutoRebootWithLoggedOnUsers and set its value to 1
If you really don't like messing with the registry and happen to be using the Professional/Enterprise edition, many configuration settings can be tweaked via the Group Policy Editor (gpedit.msc).
More info.IMPO, gpedit is one of the greatest benefits of the Professional and higher editions of Windows7, along with a free Windows XP virtual machine (XPMode) and virtual hard drives support.