As of today, my Windows 8 system is displaying a large banner covering about a quarter of the screen, demanding that I upgrade to Windows 8.1 for free. The banner helpfully informs me that that I can continue to use the computer while the upgrade is downloading. There's a button to go to the store for the upgrade, but no other choices, such as No Thanks, Maybe Later, or Absolutely Not.

I can't find a way to dismiss the banner, and I can start programs but can't give them focus while the banner is present. So I'm dead in the water. How can I fix this? Bringing up Task Manager doesn't give me any options.

My employer doesn't yet support Windows 8.1 (which is also an issue, but beyond my control) so I won't be able to connect to work if install the upgrade. So there is no way on God's green earth that I'm going to install this upgrade now.