I have solved the problem. The method I've tried is using another usable key to validate the product. And also at the same time, I found one issue related to Expression. Here is the suggestion for the will-be Windows Phone 7.1 app developers.
If you want to install Windows Phone SDK 7.1 into your Windows, be sure to first remove the Expression Blend. Because the Windows Phone SDK 7.1 comes with a Expression Blend installation that might override your original Expression Blend and also
the product key. Maybe this is the reason it keeps asking for activating the Expression product.
Be sure to install Windows Phone SDK 7.1 first and then the Expression products.