Win XP: Prevent reboots

Click Start, then Run. Then type regedit and press Enter. Be careful in the Registry, following the directions slowly and reading twice before clicking once. With the Registry Editor opened, click and expand HKEY_LOCAL_MACHINE\SOFTWARE\Policies and finally Microsoft\Windows. Right-click on Windows and select New\Key with your left mouse. Type WindowsUpdate and press Enter. This will create a new folder.
Now right-click on that folder and create another new key, this one named AU, and press Enter. AU stands for Auto Update. Turn to the right-hand pane, where an entry reads (Default); right-click on the white space and select New DWORD Value. Enter the string NoAutoRebootWithLoggedOnUsers, and press Enter. Now double-click that entry and change the Value Data to a 1, with the Base button of Hexadecimal selected.
Click on File | Exit, after which the Registry autosaves (just as other applications ought to but don't), and reboot. Once Windows loads this value it should never force a reboot when you are not present.