This is so simple. I was also having this problem.
Just right click on "My computer" and the click "Manage" . It will open a window and in that window you will see a text that says "Services" in the left column. Click on that and find out "Windows Update". Double click on that to open the properties option for that service. And there you will see a option that says "Startup type" . From that dorpdown option just select "Disable" and apply the changes and you are done. Check the screenshot here .