Here's how to prevent Automatic Update from rebooting your machine:

1.Click: Start -> Run
2.Type: gpedit.msc
3.Expand: Local Computer Policy / Computer Configuration / Administrative Templates / Windows Components / Windows Update
4.Double-click "No auto-restart for scheduled Automatic Updates installations"
5.Select "Enabled", then OK. Close the Group Policy configuration program.

Please Note the following:

1.You need Administrator priveleges to make this setting.