1. Changing the folder icon, I think, is not a standard Windows procedure. How did you (try to) do it?
2. Apparently, you messed up something by doing it. So go back to when it worked. You don't tell us your OS, so I can't be more specific. In XP, go to your last System Restore point. In Win 98, boot into MS DOS, type scanreg /restore and choose a date prior to the onset of the problems (5 days maximum, I'm afraid). Or maybe you've got another backup of your Windows folder (DriveImage, Ghost) or possible Goback installed?
3. If #2 doesn't work, download the version of TweakUI for your OS (find it with google) and try the "repair associations" option. If you're lucky, it works, but I prefer #2, if possible.
4. If all else fails, and nobody comes up with a solution, either
(a) learn to live with it
(b) try if an Explorer-alternative like Powerdesk (can be downloaded free from www.v-com.com) still works
(c) format hard disk and reinstall Windows and everything else

Kees