PCs usually last a couple years then you cant update the software anymore
http://windows.microsoft.com/en-us/windows/lifecycle
Windows XP
Released
25th October 2001
End of Support April 8, 2014
Vista (business version at least) 8 November 2006, home users got it in February the next year
End of Support
April 11, 2017
Windows 7
22 October 2009
End of support January 14, 2020
Windows 8
26 October 2012
January 10, 2023
and beyond that my fairly modest (if you ignore the second graphics card anyway) 2007 PC (
http://gbatemp.net/threads/new-pc-time.42858/ ), which by the way I ran into the ground from the point I built it and only finally kicked the bucket the other month (if you want I will look up the number of hours my hard drive was powered as that will be a good idea of how long it went for), would probably still be able to run Windows 8, it could certainly handle windows 7 (the UEFI stuff might have troubled it but I am not sure) but with the end of XP I decided to go Linux on that one. Now a machine built in 2001 is not going to handle things but that is somewhat besides the point.
Oh yeah support life cycle for OSX
OSX 10.6
August 28, 2009
Security updates stopped in September 2013, apparently it was still "supported" until February the next year.
OSX 10.7
July 20, 2011 if your machine could handle it and technically there were some 2006 X86 models that would have cost you $900 back then (about $1050 in today's money) that would not.
Unsupported from October 2014
10.8 (July 25, 2012) is now only security fixes, the end of support thing for MS above was security fixes only for the last few years though it was still 2009, 2012 and 2015 for XP, Vista and 7 for the end of new features part of that support which is still reasonably respectable).
Technically 10.9 (October 2013) is free for those on at least 10.6 and with hardware still capable of running it (the free thing was not unexpected but all those before were paid upgrades of various forms). but if you are going to make the length of support argument you ought to consider the facts.