If you have a valid Windows 7 retail key, Microsoft is now allowing you to download Windows 7 ISOs. This is legitimately a big deal as ISOs are far easier to store and keep multiple copies off than say, duplicating your original Windows 7 DVD.
There's a catch, though. Only owners of full retail copies of Windows 7 are given the option. So if your Windows version is an OEM version then best you can do is back it up or hope the factory image won't disappear on you in a hard disk crash.
Easy, but with hiccups
All you need to do is enter the retail key that came with your box and wait for the download to finish. Sadly, Microsoft isn't down with torrents so you'll just have to wait for the download and hope it doesn't cut out. Hello, download manager plugins.
According to CNet, some retail keys might now work and using keys from Microsoft's TechNet or MSDN might also be unsuccessful.
If for some reason you want a backup copy of Windows 8.1 (we seriously have no idea why you'd do that), Microsoft also has a download site.