You can do this with any icon, not just the icons on the desktop. Here's how:
Find a good replacement icon you'd like to use. A good place to find free icons is
interfacelift.com. After downloading some icons to your computer, right click on one of them and choose "Get Info". A box will pop up giving you information about your computer. In the upper-left corner of the window, you'll see a small preview of the icon. Click on it, which selects the icon. Copy it (command + c, or Edit > Copy). Next, go to your Desktop and right click the image of your hard drive. Select "Get Info" and again, click on the preview image of the icon in the upper-left corner of the window. Now, instead of copying the image, paste (command + v, or Edit > Paste).
You now have your new icon!
Alternatively, you could download a program like
CandyBar, which will change whole themes of icons for you.
Edit:
I should've considered that route...