Follow Windows Reinstallation Guide/A Clean Install of Windows 7:
Use the Activation Backup and Recovery Utility to backup your product activation on your original install. Then clean install using a Windows 7 64 bit Professional .iso and skip input of your product key. Use the Activation Backup and Recovery Utility to restore your OEM product activation. The guide explains all these steps aswell as driver installation in more detail.
Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide