Microsoft has backtracked over its decision to finally kill off Windows XP, and in a move to reassure IT departments about their Windows upgrade path, has extended its availability to 2011.

Problems with the introduction of Windows 7 were highlighted by Gartner analyst Michael Silver earlier this week, when he attacked Microsoft's decision to let computer makers sell PCs with Windows XP for only six months after Vista's successor debuts.

Silver blasted the idea as a "real mess," and said that it would make it more difficult for companies to manage their PCs, and more expensive to upgrade them to Windows 7 down the road.

Because of Windows 7's 22 October launch date, the six-month cap meant that OEMs would have to stop shipping PCs "downgraded" from Windows 7 Professional or Windows 7 Ultimate to Windows XP Professional at or around 22 April, 2010.

That policy put enterprises in a bind, Silver argued. "For an organisation that's trying to skip Vista, that means they really need to buy new PCs that they need to run on XP, and want to upgrade later to Windows 7, by 21 April, 2010," Silver said.

"[But] since a lot of organisations won't be ready for Windows 7 until later in 2010 or even early 2011, any PCs they buy from 22 April 2010 on, and until they are ready to deploy Windows 7, would need an upgrade licence or [Software Assurance] to allow them to run Windows XP temporarily, and upgrade to Windows 7 later on," he said.

The alternative, said Silver: After April 2010, companies that wanted to stick with XP for a while longer would have to buy new PCs with Vista Business or Vista Ultimate, which do have downgrade rights to XP, then downgrade to the old OS. Later, those companies would have to buy an upgrade licence from XP to Windows 7, essentially paying twice.

Yet hours after Silver blasted the plan, Microsoft backed off the six-month limit, and confirmed a new policy.

"Windows 7 Professional and Ultimate customers will have the option to downgrade to Windows XP Professional from PCs that ship within 18 months following the general availability of Windows 7 or until the release of a Windows 7 service pack, whichever is sooner, and if a service pack is developed," a company spokeswoman said in an email.

"This is good," said Silver of Microsoft's new plan. "It proves that Microsoft listens to their customers. They have changed licensing decisions in response to customer demand before, and hopefully they will do it again, because this is still not great."

Silver said his problem is with the possible "out" that Microsoft gave itself. "The new policy is 18 months or SP1 delivery, whichever is sooner," he said. "It means that if SP1 shows up in six or eight months, the date suddenly moves in."

Many software vendors may not release Windows 7-specific versions or officially support the new operating system for a year or longer after Microsoft rolls out the OS this October, Silver added. Microsoft's movable deadline for the end of downgrades could put some enterprises in an awkward spot.

"Organisations waiting on a critical vendor to support their product under Windows 7 may still have a problem," he said.

A better solution would be a hard deadline, Silver said. "The policy should be 18 months or SP1, whichever is later, or just have a reasonable date for the end of the downgrade right, like 31 December, 2010," he said.

Assuming that Microsoft holds off delivering Windows 7 SP1, it's possible that OEMs could be selling XP-equipped PCs as late as April 2011.

That's unlikely, however, as Microsoft typically rolls out the first service pack for a new OS much sooner. It released Vista SP1 to most users in March 2008, less than 14 months after Vista's retail availability. Windows XP SP1 appeared even sooner, in September 2002, a little more than 10 months after XP's launch. Windows 2000 SP1, meanwhile, was released just six months after that OS shipped.

Microsoft also slipped in a reminder about XP's limited lifespan in its emailed announcement. "Windows XP is currently in the extended support phase and Microsoft encourages customers to migrate to either Windows Vista or Windows 7 as soon as possible," the spokeswoman said.

As per its lifecycle policy, Microsoft will officially retire Windows XP, halting all patch development, including security updates, in April 2014.