If You are using Windows then notice that in fact I give You a high resolution PNG source and You have to make it usable. It means You should build an icon in ICO file format.
The best way is to use IcoFX. With bath option creation is very easy and it will give a very good result.
How to change an icon? It is easy:
1...Open Windows Explorer
2...Drill down to select desired Icon folder
3...Drag desired folder to be changed to Desktop
4...Right Click to select Properties
5...The Customize Tab will now show up like used to in Windows XP (For some reason will not show the Customize Tab within Explorer)
6...Select Customize Tab
7...Select Change Icon in Folder Options
8...Change Icon as desired from standard list
9...lick Apply
10...Drag changed icon back (Most Important) into Windows Explorer to previous location or other if desired.
11...Your Icon should now be changed and easily identified within Explorer.

If it doesn't work ask uncle google.

If You are using a LINUX (I am using Ubuntu 10.04) it should be less complicated (PNG file is enough and ICO file is useless).
Just right click on Your folder and type properties.
On the left side You shoul see an icon prevew - click it.
In the new window find You icon location and Your icon.
Click OK.
If You place Your icon in the folder You wish to customize, then You just click an icon, because an icon locator opening in that very folder, You wish to change apperance).
Good luck and feel free to ask me or google if You will experience any problems. I can say that the final effect is very good and it is worth of work You must do. Besides all things You can learn using some programs or options should usefull someday ;D

no problem.

no problem.

no problem