1 Answer

You have to purchase it first. 

Instructions
1
Locate your Windows 7 product key. It is commonly printed on a sticker affixed to your computer tower, on the software package, or in the confirmation email you received if you purchased the product online. The product key is used to activate your copy of Windows and verify that it is genuine. Without a valid product key and activation, you will not be able to receive all available updates and product support from Microsoft.

2
Insert the Windows 7 installation disc in your DVD drive and boot your computer. If your computer does not have a DVD drive and your installation files are on a USB flash drive, connect this drive to an available USB port on your computer.

3
Press the key specified on your computer startup screen to enter your BIOS. This is commonly "F1," "F2," "F10" or "Del," depending on your BIOS chip. Consult your motherboard manual if you are unsure about what to press or you don't see the information displayed as your computer boots.

4
Change the boot order of your computer in the BIOS to place your DVD drive (or Flash drive) first. Save the changes to your BIOS and then wait as the computer restarts. Press any key when the message "Press any key to boot from CD/DVD" appears on your screen. Your computer will automatically boot from the Windows installation media and display a message that "Windows is loading files."

5
Wait for the "Install Windows" screen to appear, and then select your language, time and currency format as well as keyboard or input method from the drop-down menus. Click the "Next" button to continue, and then click "Install now."

6
Read the license terms and click the checkbox next to "I accept the license terms." Click the "Next" button to continue.

7
Click "Upgrade" if you have a previous version of Windows installed and want to keep your files, settings and programs. Click "Custom" if you want to install a clean copy of Windows. The latter option does not keep your files, settings and programs, so use with caution.

8
Select the drive where you want to install Windows 7 and click "Next." Windows starts copying the files to the drive. Your computer will restart several times during the installation process, so do not be alarmed when this happens. Once the installation is complete, the message "Setup is preparing Windows for first use" will appear.

9
Enter a username and computer name on the "Set Up Windows" screen that appears and click "Next." The username pertains to your account and the computer name distinguishes your computer on the network.

10
Enter a password, and then retype the password to confirm it is correct. Type a password hint as a reminder if you forget your password. Click the "Next" button.

11
Type your product key in the space provided and click the checkbox next to "Automatically activate Windows when I'm online." Click "Next" to continue with the setup.

12
Click "Use recommended settings" when prompted to select how to handle the security and updates of Windows. Select your time zone from the drop-down menu and set the correct date and time. Click "Next." Follow the wizard prompts to set up your computer network. You can choose between a "Home," "Work" and "Public" network, depending on your network location. Windows applies the correct settings automatically.

13
Wait for Windows to finalize the installation. Once the process is complete, the "Welcome" screen is displayed and you can begin using Windows 7. Enter your username and password to log in to your account. You can now use the installation discs or the Web to install the drivers for the hardware and peripherals connected to your computer.

Read more: How to Install Windows 7 | eHow.com http://www.ehow.com/how_5536691_install-windows.html#ixzz28fml1Y6V