Windows XP first hit retail shelves. It's even more astonishing when you realize that it was still the most popular operating system in the world until the beginning of this month. The sun may finally be setting on the stalwart OS that has powered countless home and business PCs (it crossed the 400 million mark way back in 2006), but it's still number two -- right behind it's youngest brother Windows 7 and well ahead of the black sheep, Vista. Sure, our relationship with Microsoft's OS has had its ups and downs, but it's clear we've developed an attachment to the ol' bird. After all, consumer demand kept it shipping on PCs until late 2010 and Redmond has pledged to support it until April 8th of 2014. If nothing else, XP will be remembered for its incredible resilience.