if you have the product key then,
Open Windows Activation by clicking the Start button Picture of the Start button, right-clicking Computer, clicking Properties, and then clicking Activate Windows now.?
If Windows detects an Internet connection, click Activate Windows online now. Administrator permission required If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
Type your Windows 7 product key when prompted, click Next, and then follow the instructions.
if you do not have the product key then,
you can download the tools from internet to active your windows, for windows 7 you can try windows 7 Loader or windows 7 activator.