See my unofficial solution here which instructs you in downloading Windows 7 Home Premium from Digital River, Microsoft's online partner for Windows Vista/7 and Office 2007/2010.

Regarding product activation you can go through the hassle of calling Microsoft via the automated phone activation however I recommend using the ABR program and Dell files which will apply the Dell SLP activation of the factory settings and the Reinstallation DVD and does not require a call to Microsoft. I explain in more details in the video:

Download links and more information are in my guides:

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/windows-7-sp1-iso-download/

For best results also see A Clean Install of Windows:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/

In order to achieve maximum performance install a SSD such as a Crucial MX100.

Also once you have clean installed, got all the Windows updates and installed the system drivers be sure to install the latest version of Dell Backup and Recovery which will make new factory settings of this clean install.

A Clean Install of Windows goes through all of this in more detail.