“This software has only been tested and known to work under Windows XP. In order to work, your computer must not have been rebooted after being infected.
Please also note that you need some luck for this to work (see below), and so it might not work in every cases!”
However, wanakiwi claims to have tested it successfully with versions up to Windows 7, but points to some alternative information. WannaCry — Decrypting files with WanaKiwi + Demos
Dan Goodin for Ars Technica: Windows XP PCs infected by WCry can be decrypted without paying ransom – “Decryption tool is of limited value, because XP was unaffected by last week’s worm.”
John Leyden for The Register: There’s a ransom-free fix for WannaCrypt. Oh snap, you’ve rebooted your XP box – “Sooo… that’s not gonna work for you mate”