Download the Win10 iso here: http://www.bleepingcomputer.com/forums/t/584436/download-windows-10-disc-image-iso-file/ . That site has the link Aura just posted, but also a live Youtube tutorial of the download, which might prove helpful to watch. You can download as usb or DVD iso. Suggest you do both, in case one of the two media, dies.
Once you run the download tool, be sure you do NOT select 'Upgrade this PC', but rather 'download for another PC', or you cannot get the iso separately. To get the 64-bit download, you have to be on a 64-bit machine. You can get more than one version also, but no version will activate without the product key you already have in your OEM Win7 (which should be at the bottom or back or in the battery compartment of your computer), or on your 8.1 laptop. ('In the battery compartment, usually. Very inconvenient.) You will have to type in the key, cannot paste it. So write it down and keep it safe. In your case, TWO keys, since you have TWO systems eligible for one each, Win10.
The 'version' has to match what you have. If you have Win7 starter/Home, then you can only get Win10 home for that machine. If you have Win7 Pro, then you can only get Pro. If your Win7 is 32-bit, then the installer will only install 32-bit; if you have 64-bit, then the installer will only install 64-bit. For your 8.1 machine, same idea: replace '7' with '8.1' for your 8.1 machine in the prior sentences of this paragraph.
That will serve for your reinstaller. If you didn't already reserve for your Win7 copy, you can use it for installing Win10 to your Win7 machine too, but see here for getting ready, first.
Since you already reserved your copy for Win 8.1, the download will notify you when it's done. Might be any day up to some months away.
As for how long free, it's free. If you wait to install it after July 29 2016, it won't be free anymore. So if you install it prior, it stays free, though unlike others I'm not sure the updates will stay free; there's no promise of them being free, in the licensing agreement. The original Win10, yes stays free if you install it in time.
Edited by brainout, 29 July 2015 - 08:03 AM.