I think I got its resolution. I found Windows Product key using tool provided at following URL. It can read BIOS...[URL]  I used this key to activate windows and I got it activated. Now question is how key of laptop BIOS got changed... something for Dell Engineers to think about.
I brought my dell xps13 in America last July. Now i am in China. The laptop was brought in a Microsoft Shop so it must be a legal copy system. However recently, it tells me that the windows permit will expire and let me to activate it, I don't have the secret key now. My laptop service tag is and the express service code is . can you tell me how to activate it or give me the secret key? 
Windows permit expire?