Thanks Arc, for all your help up till now.
And thank you Shawn, I'll try restoring the system files if all else fails. I can simply reapply my theme/settings again. Although I might just keep the desktop icon the way it is, if nothing bad is going to happen.
I'm pretty sure though, that I didn't replace any of the original icons, just redirected the item in the placebar/ALT+TAB to use a different icon instead of the default Desktop icon, either by regedit or right clicking and properties -> change icon.
Thanks everybody! It was worth a shot.