Yip, download official ISO. Then how to do it depends on SLIC in the BIOS.
If it is SLIC 2.1 for Windows 7 by example:
1. Install Windows skipping entering a key.
2. Download Dell OEM certificate:
https://docs.google.com/open?id=0Bxj5NEo7I3z9dWx3VndfenZBWVE Copy certificate file to the root folder.
3. Download Dell OEM key:
https://docs.google.com/open?id=0Bxj5NEo7I3z9djEzRVZyd1ljQ28
4. Issue the following Windows internal commands and wait few seconds between for UAC pop up.
slmgr.vbs -ilc C:\DELL.xrm-ms
slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxx-xxxxx
(replace xxx's with Dell key)
5. Now check activation status by entering:
slmgr.vbs -dli