RE: New Dell Bios rendered XPS desktop unusable
You might try creating a bootable USB with the Win 7 file directly from Microsoft and try that version, which is probably more up-to-date than the one you got from Dell's site. But you need your Microsoft Product key from the sticker on the tower to start the download from Microsoft.
Dell's Windows 7 Reinstallation .isos are updated to January 2016 and include addition driver support such as USB 3.0 and storage controllers for up to Skylake systems. They also apply Dell OEM SLP.
Microsoft's on the other hand are seriously outdated December 2010 moreover require a Windows 7 key which one may not have if they are invoking downgrade rights and in any case it rejects Dell OEM keys.
Stuck at the "Starting Windows" screen. This usually occurs when hardware is faulty... Since you tried a Reinstallation. Are you currently stuck on the first Starting Windows Screen (when you boot from the installation media and then select your language options etc) or the one after it restarts?
This can be a memory module but Dell F12 preboot diagnostics should pick this up.
Sometimes it occurs if there are USB storage devices such as external hard drives plugged in or a card in the card reader so as Ron mentioned remove anything apart from the keyboard, mouse and bootable installation media.
Since this occured after a UEFI BIOS update also have a look at your UEFI BIOS settings. Its most likely that the UEFI BIOS update has changed a setting and this is preventing the start up.
UEFI BIOS settings:
- The Boot Mode should be set to UEFI.
- Secure Boot should be Off.
- Legacy ROMs settings (not shown on the Boot Manager Screen) should be Enabled.
The installation media should be set to the GPT partition screen.
Check that your SATA Operation is AHCI. If you have a small 32 GB M.2 SSD cache drive then it may also be useful to remove this to check if this influences the Boot.
Check also what the settings are particularly for your Ethernet controller, Wireless Card and other hardware. It may also be worthwhile going through and disabling hardware in the UEFI BIOS setup one by one and seeing if you can pass the boot screen. I can't give you the screenshots of this as I don't have an XPS 8900.
Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide