I agree it's awfully frustrating to be trying to work and suddenly windows starts performing updates. It really slows down your computer while performing those updates.
The answer is to go to Control Panel in Windows 7 or Windows 8 and then search for in the little search box in the upper right-hand corner "windows update". Once you get the windows update information showing choose change settings and there you'll find an option to set the time at which the automatic updates occur.
You can also set options for that you want the updates to automatically be installed or only give you a warning about the new updates. Some people like to have the updates automatically installed and for most people that's probably the best option. You may decide to choose the option that allows you to control when the updates get applying and you have a chance to verify that these updates are something that you want to apply. For most people automatic update option is best and you can select the time at which those updates will be applied.