Windows is dead. Microsoft is going out of business.

So goes the common refrain in Silicon Valley.

And to be sure, the Windows business looks worse than it has in a long time, as many consumers put off PC purchases because of the economy, or opt for iPads instead.

Earlier this week, Gartner downgraded PC sales expectations for the third time this year. PC makers like HP and Acer are suffering or getting out of the business. Microsoft's Windows revenue has been shrinking on an annualized basis for the last two quarters. The iPad now has 11% of the PC market.

Next week at its BUILD conference, Microsoft will unveil more details about Windows 8, the operating system that is supposed to beat back the iPad while still maintaining a lot of what made Windows so popular in the first place. Skeptics doubt Microsoft will be able to pull it off.

Maybe. But Windows has been threatened many times before, and somehow it keeps surviving....