Windows activation key

Hello!!!!
I bought your laptop DELL Instiron 7737. The purchase was make 12/23/2013.
Until now there were no problems and did not attach any importance to some small things.
The thing is that with your computer did not have the Windows installation disc.
Moreover, nowhere specified key for Windows.
For this reason, I can not activate the system after recovery.
What do I do in this setuatsii: buy a new license or have another way out full of?

Best regards

Valeriy Kurnosenkov