I downloaded Windows 8.1 from [dreamspark.com](http://www.dreamspark.com). While I was downloading the file it showed something like a 24 month extension guarantee and also an expiry date in 2015. What does this mean? Will my Windows 8.1 license expire after 2015 or can I use the OS in the same way I use my Windows 7 (perpetual license)? I am pretty new to this stuff, please excuse me if you feel this is silly.