If you have a 7 or 8 system you intended to upgrade for free to 10 then be warned Dec 31st is the last day to use this workaround
http://www.zdnet.com/article/microsoft-quietly-announces-end-of-last-free-windows-10-upgrades/
http://www.zdnet.com/article/microsoft-quietly-announces-end-of-last-free-windows-10-upgrades/