Microsoft is offering users a compelling deal to switch to its upcoming Windows 8 system: Those who already use Windows XP, Vista or 7 can get their hands on the professional version of the software for just $40. Well, $39.99.

The deal, according to Microsoft, will be good in 131 markets.

While the price isn’t quite as good as Apple’s upcoming $19.99 upgrade to Mountain Lion, though still better than the current Windows 7 upgrade price of $199.99.

As you might expect from such a low price, the $40 upgrade will only be available online as a download from If you prefer a disc, you can also pay $15 for a backup disc or just buy a $60 upgrade version in Microsoft stores.

This provides a clear incentive for Microsoft users to make the leap to Windows 8, particularly for those who never left the comfort of Windows XP. According to the analysis site NetMarketShare, about 40 percent of computers are still running Windows XP, just 1 percent ahead of the 39 percent market share from Windows 7.

Users running XP will only be able to take their personal files with them through the Windows upgrade. Windows Vista users will be able to bring their Windows settings and personal files; Windows 7 users can bring their settings, files and apps.

If you want to reformat your hard drive before upgrading, Microsoft said, you can do that from within the setup wizard that comes with the upgrade download.

Microsoft is making a big bet with Windows 8, which is supposed to help the company keep its PC sensibilities while making its push into the tablet market with the introduction of the Surface.

The company has to fight not only for tablet share, but also to boost growth in the PC market, which has returned to growth but very slowly. Growth rates for Windows PCs are particularly small, as Apple has managed to stay ahead of market growth. According to a chart shared Monday by Asymco analyst Horace Deidu, the ratio of PCs to Macs is the lowest it has been since 1985.

On Monday, Microsoft announced that it will take a $6.2 billion hit on its fourth fiscal quarter by writing down the value of its ads acquisition, aQuantive. According to a company release, the acquisition “did not accelerate growth to the degree anticipated,” prompting Microsoft to lower its value. Analysts had projected a $5.3 billion profit for Microsoft — now it may face a loss on its books.

But analysts say it’s not a good idea to count Microsoft out quite yet. With the content it has on the Xbox, its push into mobile, growth in search with Bing and the possibility of a hit with Surface, Microsoft’s not out of any game quite yet.

Or, as Apple chief executive Tim Cook said in an earnings call in January of the mobile market: “There’s a horse in Redmond that always suits up and always runs, and will keep running.”

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