If you want to customize the appearance of your Windows PC, the first few elements you must change are the desktop wallpaper, the screensaver and of course, icons. Customizing icons is quite easy: simply right click on the correspondent file, select Properties and click on the Change Icon button. You'll open a dialog window where you'll be able to select a new icon for your file.
However, this works only for shortcuts. If you want to change folder icons, then you have to right click on any given folder, select Properties and click on the Customize tab. Here you can choose between selecting a picture to spice up the folder icon (and also make it more recognizable) or changing the icon altogether.
But if you want to change system icons (that is, those that appear on your desktop labeled as My PC, My Documents, My Network Places and the Recycle bin) you have to follow a different route: right click anywhere on your desktop, select Properties and click on the Desktop tab. Then click on the Customize Desktop button and you'll open a window where you can select which system icons to display and customize them in a very easy way.
Finally, don't forget that there are many software programs that can help you customize icons in a variety of ways, such as these icon apps collected by Nick or the bunch of folder coloring tools I wrote about a while ago.