Cause 1: You have installed virus-protection software that guards against modification of the System file.
Solution 1: Start up your computer from the Mac OS CD.
Solution 2: Drag any virus-protection software out of the System Folder and restart your computer. You can put the virus-protection software back after installation.
Cause 2: The System file or the Finder file may be locked.
Solution 2: To see if your System file or Finder file is locked, select the file's icon. Then open the File menu and choose Get Info. Make sure the Locked checkbox is not selected.