Thanks for this, user57, I've been searching all over for a way to manage HEIC on XP! On XP Pro x64, I tried the decoding on an iPhone picture, both jpg and png, but sadly it ended up with random line artifacts throughout the picture. I haven't tried it on x86 yet.
If it's ok to mention here, I have found another free program that just recently started handling HEIC on XP too. It has an editor, viewer and converter!