In windows 7,only Windows Explorer was pinned to taskbar by default,if you want replace it with My computer,please follow the below step by step guide to pin My Computer to taskbar in windows 7
1.Right-click a empty place of you desktop,select New—Shortcut
2.Type in the following line and then click Next
3.Type in the name for this shortcut,then click Finish
4.Go to your desktop and right click this shortcut and select Properties
5.Go to Shortcut tab and click Change Icon
6.Type in shell32.dll in then press enter then select a icon for this shortcut,then click OK twice.
7.Go to your desktop again,right click shortcut “My computer” and click Pin to Taskbar
8.All done,now you have pin My computer to taskbar and you can remove the shortcut from desktop.