Disable Secure Boot

You will need to disable Secure Boot in the BIOS setup if you wish to install another Operating System.

1. Power down your computer. Wait 10 seconds and power it up. Press F2 at the Dell BIOS Screen.

2. This will take you to the BIOS setup.

3. Press the [→] key and then the [enter] key to select the Boot tab.

4. Press the [↓] key until you to Secure Boot and press [Enter].

5. Press the [↓] key until you get Disabled and press [Enter].

6. Press F10 to Save and Exit. When prompted select yes by moving the [←] key and press [Enter]

7. The system will reboot.

8. Follow the installation instructions in A Clean Install of Windows 7.

If you still cannot install Windows 7 with the SATA drivers, disable UEFI boot also (see here).