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Assuming you are running Windows XP on the system in question, make sure you are logged on as a user with Administrative privileges.
If you are not sure if you are logged on as an Administrator you can try this simple test:
How to tell which type of account you're using now
If you're using Microsoft Windows XP, an easy way to tell whether you're using a Limited User or Administrator user account is to move your pointer over the clock on the bottom right of your screen. Right-click the clock, and then click Adjust Date/Time.
• If you're using a Limited User account, you will get a message reading "You do not have the proper privilege level to change System Time."
• If you are using an Administrator account, a small window will open with controls that enable you to change the time and date on your computer.
If you are not logged onto an Administrator account, reboot and log in as "Administrator".