Ok, if its downloading its too late via this method.
You may not actually be able to stop the donload (but it will still not install, that you have to allow)
But this trick can work to stop it
First, download the patch from my link belowhttp://www.microsoft.com/downloads/details.aspx?FamilyId=a5713587-f0f7-4878-b81b-784755a7c03f
Open up your registry editor using the Start Menu search or by pressing WIN + R and typing regedit and hitting enter, and then navigate down to the following key:
You’ll probably have to create the WindowsUpdate key on the left-hand side, which you can do by right-clicking the Windows node. Click on that new key, and then create a new 32-bit DWORD called DisableOSUpgrade on the right-hand side, and give it a value of 1.
Then restart the computer and tell me if it still downloads please
I can also do these steps for you, and have made a request via Additional Services. This is optional, and if you choose to do this, I can, with your permission, remote into your computer while you watch and do all these steps (and deal with any problems that arise during them) for you. Or if you do not choose to take this service, we can continue here. This is no problem at all. Just let me know please.