2. Patch the critical vulnerabilities under active attack
Microsoft could selectively patch only the critical bugs that are being exploited by hackers. Presumably, that would be a subset of the complete XP patch collection assembled each month.
Some analysts have picked this option as a possibility. Last December, Michael Cherry of Directions on Microsoft posed just such a situation.
"Suppose ... a security problem with XP suddenly causes massive problems on the Internet, such as a massive [denial-of-service] problem?" asked Cherry at the time. "It is not just harming Windows XP users, it is bringing the entire Internet to its knees. At this time there are still significant numbers of Windows XP in use, and the problem is definitely due to a problem in Windows XP. In this scenario, I believe Microsoft would have to do the right thing and issue a fix ... without regard to where it is in the support lifecycle."
3. Charge users for XP patches
Although Microsoft would much rather book revenue from the sale of a newer OS, it may realize that some will refuse to upgrade, and try to make money rather than give away fixes.
It's unlikely that Microsoft would be able to charge $200 annually for post-retirement patches, as it does with Custom Support customers, but it may be able to get away with $50 a year for individuals and small businesses, perhaps with a maximum machine cap at, say, five PCs per customer.
Traditionally, Microsoft's not charged for support, but it could cast this as a special situation caused by the longevity of XP, which was due to the delay of Vista and secondarily, that OS's subsequent flop. In late 2007, when Microsoft extended XP availability to OEMs by several months, it cited Vista's delayed launch for the unusual move. (It added another extension in 2008 that kept XP alive on new "netbook" PCs, the then-popular class of cheap laptops, until mid-2010.)
And Microsoft has talked up a transformation to a "devices-and-services" company; a pay-for-support plan would mesh nicely with the latter half of that strategy.
4. Heavily discount Windows 7 or Windows 8.1 to XP users
For several months late last year and through January 2013, Microsoft sold Windows 8 Pro upgrades for $40: It has not revived the cheaper prices since.
Microsoft might try another discount to nudge XP users off the creaky OS, pitching them either Windows 8.1, the update slated for a mid-October debut, or less likely, the option of moving from XP to Windows 7.
The latter would violate Microsoft's standing policy of shutting down retail sales of the preceding edition a year after the launch of a successor, but it might be worthwhile to backpedal to squeeze some money out of the XP situation without facing the backlash when customers complain that they're being pushed to adopt the radically-changed Windows 8.1.
Some revenue, in other words, would be better than no revenue, even if Microsoft had to eat crow and offer Windows 7 as an option.