Since Vista arrived Windows has been clamped down somewhat in what you can do as a user now, even as an "Admin" user. You’ll find that often you are told you need Administrator Privileges when trying to run applications and this also happens when trying to run some commands from the command prompt. This is a step-by-step guide on how to open a Command Prompt with Administrator Privileges so that you can run any commands you may need.
the Start Icon
or Button and then click All Programs
Then click Accessories
Now Right Click on Command Prompt
and select Run as administrator
You will now be faced with the Windows UAC window. Simply click Yes
You should now find yourself looking at the black Command Prompt window with administrator privileges as indicated by the "Administrator:" start of it’s title bar.