It turned out to be, press F12 at the POST screen, as it states in the bottom, then select the optical drive when the boot device menu appears. It seems to have worked. To prevent all this from happening again, I need to go to System -> Administration -> Software Sources, and uncheck the Update tab. Hopefully it will be OK now. Sometime in the future I may learn to use Updates selectively, or install DKMS, which is clearly not especially easy. If I get it online tomorrow, I shall drop you all an email from it. Thanks for all the advice ...
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