Have you ran the diagnostics on your unit? Power it down and hold F12 as you power it up, run the diagnostics and any extended tests.
You should have a Windows 7 COA sticker on the base of your system or under your battery compartment. Providing you can read it, you should have a 25 digit product code.
Take a note of it. I will take you through the reinstallation procedure using retail media.
As you can see in my sticker above, it is for Windows 7 Home Premium. You will need to check your version on your COA likely it will be Home Premium on this model. Take the 64 bit version of Home Premium. Next you may download Windows 7 from Digital River and create a bootable USB for reinstallation. The instructions for doing this and the download links may be found in my wiki Official Microsoft Windows 7 .iso Downloads (Digital River). Once you have this bootable USB you should follow my complementary wiki A Clean Install of Windows which will guide you through the manual reinstall procedure.
Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide