And you don't have to worry about the risk of malware included with 3rd party activators or pre-activated ISOs.
I bought several copies of Windows 8 Pro when it was released for £15 each, saw me through 8.1 and now on to 10.
On the other side of the coin however, I got so fed up with calling MS about having to reactivate my Windows on my various PCs, that I ended up simply using a naughty copy purely to bypass activation, and I never once came across any iffy malware of any kind on any of my installation ISOs.
On a similar thought, you also get updated ISOs complete with the latest updates etc, and these are sometimes very beneficial to many people.
I mean... my Win 7 and 8.1 PCs are using naughty disks purely because I can install them and the updates are VERY minimal, whereas, if I use the master DVDs, there is over 1GB of updates on each of the 8.1 setups alone... Thats just not funny.