Manual Registry Hack: -
Open up regedit.exe through the start menu search box or run dialog, and navigate down to the following key, creating new keys if they don’t exist.
Create a new 32-bit DWORD value named NoAutoRebootWithLoggedOnUsers and give it a value of 1 to prevent automatic reboot while users are logged on. Delete the value to put things back to the way they were.
Downloadable Registry Hack: -
Just download and extract the registry hack files and double-click on WUNoAutoReboot.reg to disable automatic reboots. The other script will remove the hack.
Using Auto Reboot Remover Utility: -
If you’d rather not mess with the registry, you can use a small utility created by the guys at Intelliadmin which will make the changes for you. Just make sure you right-click and run as administrator if you are using Vista.
This hack should work for the professional or business editions of XP, Vista, or even Windows Server. I’d be interested to hear your feedback in the comments.