Uninstall Pre-Installed WIN 10 From XPS 8900 and Re-Install WIN 7

Hi ScandiHooligan52,

>I purchased the XPS 8900 direct from Dell.  Didn't know Win7 was available,

:-)
I've built my own PCs since the DOS days.  I feel old. :-)
Well, thanks to Microsoft, I HAD to by an XPS 8900 to be able to run Windows 7 on the latest generation of hardware.  Microsoft supports Windows 7 on the 6th gen Intel processors for only a few manufactures and only a few systems from each.
I've bought "a few" :) used (and cheap) Dell systems from ebay to use as dedicated systems.  And, most of the tech places I work have used Dell.  So, imho, that meant I would buy a Dell PC (and hack it up!) to get a 6th generation Intel system.  Note,

Back to your question:
You need to fully format and wipe the Windows10 disk. 
Windows 7 install doesn't like to delete or modify a Win10 GPT partition(section) on disk.

There are some free disk tools that will delete and format a windows10 disk.  I think that the free easeUS version may do that (I have the paid version).

Btw, Win7 can install on FAT32, NTFS, or GPT.  It can install under UEFI or legacy.
But, you need to make sure that the boot disk type and BIOS settings are the same.
If you never want to use a disk over 2TB, you can use and stay with Legacy.  Imho, that's a very bad choice.

If you haven't changed the BIOS boot/disk setup, you should be fine with a blank disk with no partitions on it.
Btw, for what ever reason, I couldn't get my modified BIOS setup to self-boot a windows7 new disk.  I had to do what was suggested in the following article:

http://www.dell.com/support/article/us/en/19/SLN245206/EN?dgc=AF&cid=285865&lid=5550010

After I did the above, the system (XPS 8900) booted fine.  And, I was able to change other settings, like enabling SMART, and have the PC self-boot.  Before that, I had to choose the disk to boot from.  Maybe I missed something in the BIOS before, it was late when I was doing that install.

Good Luck!

I just thought about your post some more.

Summary: 
Legally, you need to buy a Windows 7 license to run Windows 7 on your PC.

Microsoft sells OEM and Retail licenses to the public.
Either will work for you.

Most new unopened Windows licenses (you buy from Amazon, newegg, bestbuy, etc) are OEM  licenses.
An OEM license is valid for one and only one PC.  The license can NOT be transferred to another PC.

A RETAIL license can be transferred to another PC (such as if the current PC dies, or an upgrade).  But, RETAIL licenses are harder to find, and cost more.  A RETAIL new never opened Windows 7 is very hard to find, and not cheap.

More Detail:
Iiirc, Dell only offers Win10 Home to the general public on the XPS 8900.
If your PC came with Windows10 Home, that means that you do not have the licensing rights to install Dell Windows7 on your PC.
Will it "work" - maybe.

***  Can Microsoft detect what OS the PC came with therefore invalidate a Dell Windows7 install - ABSOLUTELY!

Just about all Windows 8 and Window10 PCs come with the software license embedded in an on board flash/BIOS.  It's not hard to read.

Windows 10 Pro OEM has downgrade rights to Windows 7 Pro OEM. 
Note that Windows 10 Home or Windows 10 Pro Retail do not have downgrade rights.

Fwiw, I bought my XPS 8900 with Windows 7 Pro already.
However, for many reasons, I need to be able to transfer my OS and software to another motherboard in case my current motherboard dies.  So, that meant buying another XPS 8900(likely used) that came with Windows 7 Pro.  Or, use one of my new unopened Windows 7 Pro Retail licenses.

So, I'm using one of my new unopened Windows 7 Pro Retail licenses for my XPS 8900.  This allows me to move my OS and software to another XPS 8900 motherboard if my current motherboard dies.
NOTE:  Before transferring the license, the old mother-board must be destroyed.
Microsoft has every right to revoke a license if it's used on two working PCs.  From what I read, they will do that also.

Fwiw, I have "a few" :) retail licenses for XP, 7, 8, and 10.  For my work, I need that flexibility.  It's called thinking and planning for the future. :)

>if my current motherboard dies.
By that I meant if a lightning strike or power surge makes it past:
Belkin surge protected power strip, UPS, 2nd Belkin surge protected power strip, my upgraded Cosair power supply, and then to the motherboard.

It is unlikely - yes very.  It the probability "zero" - no. :-P  The combined cost of my professional software is more than the cost of a new car. :-O  I do disk clones and backups regularly.  But, I need to protect my investment and work environment in case of a motherboard failure.

Good Luck!

Thanks Graystang86.  I just got home from being on the road.  I'm going to attempt what you've suggested this weekend.  Let you know. Regards! ScandiHooligan