Your question is pretty open.
If you are referring to the "My Computer" icon itself, you would use the following:
"Right-click an empty spot on the desktop > select Properties > click the Desktop tab > click the "Customize Desktop" button > select (highlight) the My Computer icon > click the "Change Icon..." button > navigate to the icon you want > click OK > click Apply"
If you are referring to all computer icons on your system, you can use the "Context Menu > Properties" for some icons to change the icon. Note - there are many icons, such as drive icons, which are no longer customizable this way in XP.
For an easy way to change many of the System icons and many file icons, you can use a third party program like "IconPackager" which allows you to apply a package of icons with a click of the mouse.
You can look under the "Software" tab at the top of the page to read more on IconPackager.
WinCustomize Forums