During installation, you can skip entering the Windows 7 Product Key–just leave the field blank (as shown above).
Windows will run normally for 30 days, but it’ll be nagging you to enter the product key in the whole time. After 30 days, it’ll lock itself down and prevent you from doing most tasks on the computer. No worries, just follow these steps:
1. Click Start and then type cmd.
2. On the Start menu, right-click cmd and then press Enter.
3. When prompted by User Account Control, click Yes to start the command prompt with administrative privileges.
4. Type slmgr -rearm, and the press Enter.
5. When prompted, click OK to restart your computer.
Windows 7 will begin nagging you again, but you’ll have another 30 days. You can only rearm Windows 7 three times, allowing you a total of 120 days. Sure, it won’t last forever, but it’s great for those test computers that you tend to regularly re-install.
After that? Buy a license! You can do it directly from Windows, online, or at just about any electronics store.