After completing windows updates on your vista you should see similar to the following popup screen for reboot.If you want to disable use the one of the following solution.
gpedit works fine on vista, just not on home basic or home premium. it’s only available on business, enterprise, and ultimate.
1 Press Windows Key + R and enter “gpedit.msc” (without quotes) then press enter.
2 Navigate to:
Local Computer Policy
3 Doubble Click on: No auto-restart with logged on users for scheduled automatic updates installations
4 Select Enabled and Press OK
5 Press File & Exit
Type cmd into the start menu search box, and use Ctrl+Shift+Enter to open in administrative mode. You could also right-click the item in the menu and choose Run as Administrator.Once at the prompt, just type in the following:
net stop “windows update”
Don’t worry, the service will start right back up when you reboot your computer, but at least you don’t have to deal with the restart dialog.