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Turn off your computer. Turn back on your computer. Press F8 repeatedly. Select Repair Your Computer. Click Next when prompted. Enter your password when asked. Click System Image Recovery. Select the factory image. Click Next. Confirm that you want to erase the hard drive.
You can turn off your Windows 7 computer by doing the following:  Click Start {"smallUrl":"https:\/\/www.wikihow.com\/images\/b\/b3\/Windowswindows7_start.png","bigUrl":"\/images\/thumb\/b\/b3\/Windowswindows7_start.png\/30px-Windowswindows7_start.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows 7 icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"}. Click Shut down in the lower-right side of the Start menu. Wait for your computer to power down completely. Press the "Power" {"smallUrl":"https:\/\/www.wikihow.com\/images\/5\/5d\/Windowspower.png","bigUrl":"\/images\/thumb\/5\/5d\/Windowspower.png\/30px-Windowspower.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Public_domain\">Public Domain<\/a><br>\n<\/p><\/div>"} button on the computer's housing to do so. Doing so as soon as your computer begins to turn on will cause the advanced startup menu to open. If nothing happens and you arrive at the login prompt like usual, you began pressing F8 too late. Restart your computer and try again. Use the arrow keys to move the highlight selection to this option near the top of the screen, then press ↵ Enter. Doing so confirms your keyboard settings. Type the password you use to log into your computer into the "Password" text box, then click OK. It's a link in the middle of the window. Click the "Dell Factory Image" (or similar) option to select it. You may have to click a drop-down box to find the "Dell Factory Image" option. It's at the bottom of the window. Depending on your version of Windows 7, you may have to check a box and then click Erase or OK, or you may need to select a hard drive to format and then click Erase. Once you finish confirming that you want to reset your computer, the computer will begin erasing your hard drive, after which point it will reinstall the version of Windows that was present on the Dell factory system image.