Hi Sebastiaan,
The ESP partition is the EFI System partition for system firmware. Dell hardware diagnostics are stored in DIAGS partition. MSR partition is a partition reserved by Microsoft. WINRETOOLS contains Windows recovery tools and PBR image has the system recovery image.
You may downgrade the system to Windows 7 Pro using Dell OEM disk. Please provide me the Service Tag and the name as it appears on the account via private message (click my username and then click Start Conversation) so I may access your records and check for further course of action.
Keep me posted with the information. I will be glad to assist you.