Well, I just got round to downloading the official .iso from MS, & guess what? Apart from the same filename & byte size [which I know on their own don't mean much], it matches hash results exactly for 3 different hashing algorithms [MD5, SHA-512, & Whirlpool] when compared to the 1 I got from an alternative source. :D

Then you're lucky ;)

of course not all alternative sources are infected, but why take the risk? :)