Namur: You sure did mate. Already repped you up ;)
The Indeo Codecs fix worked like a charm for me.
Romulus: Cheers! Hopefully Zoidberg will mark the solution so its clear for other peoples.
Firstly thanks to Namur for taking the time to re-encode them.
Well, I've tried something again. I tried extracting the files from the zip then try to read them from a non-compressed folder... and it works.
I've just re-tried that after uninstalling the indeo codec pack, and it still works (maybe I should reboot to be sure).
So for those having the problem of a popup error coming up when trying to read it under WMP (by double-clicking it within the ZIP archive), try extracting the files first.
Thanks for everyone who helped with this! ;)
edit: I wanted to mark Romulus', Namur's AND my last post as solution but we can only mark one as solution (and not mine, lol :P). But thanks again to you too for helping! ;)