There are two big options to activate it:
Activate using the Internet
Activate by using a direct connection
Open Windows Activation by clicking 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. The Windows 7 Serial Numbers, Windows-7 32-bit Serial Key Free should be like:
342DG-6YJR8-X@@GV-V7DCV-P4K27 (32 Bit)
We strongly suggest you to buy a Windows licence from Microsoft!