You should be able to download and reinstall Windows using the Dell Recovery Image:
You won't need a key if the system had a Windows license.
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E7440 doesn't come with optical drive so you would need a recovery USB stick.
The Dell OS Recovery Tool will help you download a customized Dell recovery image of Microsoft Windows or Linux that was preinstalled on your Dell PC.
Note: For instructions on how to download the operating system recovery image and create a USB recovery key in Linux, refer to the Dell knowledge-base article How to download and use the Dell OS Recovery Image in Linux.
Note: If you have the Windows 10 S version it will not allow the download or use of Dell OS Recovery Tool on a PC running Microsoft Windows 10 S. To create OS Recovery Media for a PC running Microsoft Windows 10 S, follow the instructions to create OS Recovery Media from a Windows PC and select "Other Device" at the "Select Device to Recover" screen. This is Windows 10 S applicable only.
If Windows 10:
If Windows 7 Pro: