Instructions
Log in to your Netflix account on your computer.
Click on "Your Account and Help" in the upper-right corner of the window.
Scroll down to the section titled "Watching Instantly On Your TV or Computer" and click the button that says "Manage Netflix ready devices and computers."
Locate your Xbox 360 on the list of activated devices and click "Deactivate."
Return to the "Your Account and Help" page and locate the button that says "Activate Your Netflix Ready Device." It will ask you to input a code that will be generated on your Xbox shortly.
Select the Netflix icon in the Video Marketplace menu on your Xbox 360. Since you are already a Netflix member and are simply re-setting your account sync, select "Yes, I am."
Write down the activation code that appears on your television screen or if you have a laptop, just copy it into the activation code box on your computer screen.
Enter the activation code provided by the Xbox into the yellow box on the Netflix screen on your computer. Click "Activate."