Dell does not modify the core Windows OS files. Instead, they provide a separate driver pack that you load during or after installation. Therefore, searching for a combined "Dell Windows Server 2008 R2 ISO" is searching for something that never existed.
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If you are trying to recover or reinstall this OS on a Dell PowerEdge server, here is how you can still find the media and the drivers you'll need. 1. Where to Find the ISO dell windows server 2008 r2 iso download