You must completely reinstall Windows to go back to Starter.
OEM computer manufacturers are required to provide at least one way for you to reinstall Windows if you buy a computer with Windows preinstalled - your computer should have included either a restore partition (check your manual) on your hard drive, or Windows
If you don't have either contact the OEM computer manufacturer and ask them to send you an install disk for your computer - they may or may not do it. If they agree to send you one they may charge a small fee.
Before you reinstall, try phone activation for your current install.
Call Microsoft Using Manual Phone Activation
If you are currently installing Windows - at the Enter Your Product Key page, do not enter your key and uncheck "Automatically Activate When Online" then click OK/Next to finish the installation.
If you have trouble activating Windows 7 open the Windows Activation Wizard while in Windows 7 to obtain a phone number to call Microsoft and activate Windows:
1. When you reach the desktop click Start, then in the Search box type: slui.exe 4
2. Press Enter on your Keyboard
3. Select your Country.
4. Select the Phone Activation option, then call the number given and hold for a real person.
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