Ubuntu provides five years of support for their LTS releases. The oldest OS that Ubuntu support is from 2010. Red Hat just EOLed an OS from 2003. Debian provide

Ubuntu provides five years of support for their LTS releases. The oldest OS that Ubuntu support is from 2010. Red Hat just EOLed an OS from 2003. Debian provides support for previous releases for "about a year or so". While Windows XP was released in 2002, over 12 years ago and Windows XP Embedded will be supported until 2016. So it looks like if you want the longest period of support, your best bet is Windows not Linux.