To disable the security centre: -
Open the control panel, double click administrative tools and double click services.
In the services box that opens, scroll down until you can find an entry called "Security Center" or something like that.
Double click it and another box will open. Towards the middle of it, there will be a section called Startup, with a list.
Click on the list and from the options that appear, choose 'disabled'.
Click OK and exit out of the services console.
This will disable the security center, and all of the annoying nag popups you've been getting. But remember to visit the Windows Update website frequently - there will often be a notice posted on CF when MS are about to release their monthly wave of patches so when you see it, head straight to Windows Update and get patched.
