Windows XP is the OS that won't die! Like Jason from a Friday the 13th movie, it keeps rising from the dead—just when you thought we'd finally escaped! And this week, it's happened again.

In a blog post describing this week's beta release of SP1 for Windows 7 (and Windows Server 2008 R2), Microsoft also quietly revealed that it's extending so-called "downgrade" rights to business customers, effectively ensuring that XP will be available in the marketplace until—get this—2020. That's right: It's possible that businesses could still get XP on new PCs almost 20 years after the OS first debuted.

Microsoft had previously extended its downgrade rights program—which lets businesses "downgrade" to XP from current Windows versions on new PC purchases—to some time in 2011. So how did we get from that date to 2020?

The reasoning behind this decision is a little convoluted, but the details are simple enough. According to Microsoft, downgrade rights for XP would have previously expired 18 months after the release of Windows 7, or at the release of Windows 7 SP1, whichever came first. Windows 7 shipped in October 2009, so that date plus 18 months is April 2011 (which, coincidentally, is when Microsoft now expects to deliver SP1).

Under the new terms, business customers will be able to downgrade to XP during the entire Windows 7 life cycle. And while Windows 7 Ultimate (which is aimed at consumers) will exit extended support in January 2015, business-oriented versions such as Windows 7 Professional will be supported until January 2020.

So, why extend XP's availability another nine years—and do so this late in the product's life cycle? Simple: Microsoft says that fully 74 percent of all businesses worldwide are still using XP in some capacity.

The extension doesn't apply to other versions of XP. For example, PC makers can sell XP Home Edition with netbook computers only through October 2010. And XP SP2 will still expire this week; customers will need to upgrade to SP3 to continue receiving support from Microsoft.