On XP Home how can I prevent Windows from ever trying to restart my computer after an update? XP Home has no Group Policy Editor.
The above is a great suggestion! I think it deserves this answer as a compliment however. When you have been pigeon holed into that "restart now/later" dialog box (coincidently happening to me right now) it is very annoying how it keeps popping up every so often. Sometimes you are in the middle of something and it is just not a good time to reboot until you have cleared a few things off of your "desk"
So to that end I also suggest killing the windows auto update process with your favorite taskmanager.
The process is "WUAUCTL.EXE" you will know it is gone when the yellow shield disappears from the system tray.
And besides... You will end up rebooting soon anyway,
As an alternative to Kirill's solution, the Group Policy Editor can be used if it's available (it's not available in some editions of Windows):
1) Open the Group Policy Editor (Run > gpedit.msc)
2) Navigate to Computer Configuration > Administrative Templates > Windows Components -> Windows Update
3) Double click the No auto-restart with logged on users for scheduled automatic updates installations setting
4) Set it to Enabled
5) Click OK and then just close the Group Policy Editor