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It is ICO_MYCOMPUTER.ico in explorer.exe in the Icon folder in C:\Windows
You will need a resource hacker of some kind so you can open the explorer.exe and import a new icon in its place. I would recommend getting a copy of explorer.exe and putting it on your desktop and then edit it leaving the original untouched until you rename it using the steps below.
Then you'll want to change the explorer.exe do the following.
To properly change a system file you need to take ownership give yourself permission and rename the old file and then copy in the new one look at Post #6 of this thread will tell you how to do it Basic Networking Information
Let me know if you need any more help.