Now that Windows 7 is firmly entrenched in the marketplace, the next question is how long will Microsoft support Window XP, Vista, and Windows 7? Although, Microsoft has a well-documented support lifecycle for its software products. "For Win7, you can do the math yourself. The GA date for all Win7 editions was Oct'22, 2009. 5 years after that is Oct'22, 2014. The next quarter begins in Jan'2015, and the 2nd Tuesday of that month is Jan'13. So, that's when mainstream support is scheduled to end. Extended support for business editions goes an extra five years, until Jan'14, 2020 (the 2nd Tuesday of the month)," explains ED.
"For XP, however, those calculations don't work, because Microsoft has extended XP's life [ 1, 2, 3]. To find XP's end-of-support date, search Microsoft Product Lifecycle page here's the set of search results for XP:
"SP1 and 1a were retired back in 2006. SP2 rode off into the sunset on July 13. And SP3 will be retired along with all editions of XP on Patch Tuesday, April 8, 2014.""
By that time, Windows 8 will probably be well past its first birthday, and Microsoft will be supporting four separate Windows versions. Here's a table that summarizes the support policy for all of the current Windows desktop versions:
Reference: Support Lifycycle