@JOcean 

As I've already stated, this PC came with Windows 7. But when I decided to update to Windows 10 Pro, It passed the compatibility check. I even did a fresh installation to make sure, & it was still able to install Windows 10 without any problems. I even bypassed the Windows 11 installation & so far, no issues or malfunctions have occurred as of now. So I don't understand why Dell decided to put it as Windows 10 incompatible On the Microsoft website, read the bolded words. And so far, there aren't any problems occurring. On the Microsoft page, it stated "If you proceed with installing Windows 11, your PC will no longer be supported and won't be entitled to receive updates. Damages to your PC due to lack of compatibility aren't covered under the manufacturer warranty." 

To begin with, as you know, that is quite an old CPU and Dell did not even list the 790 as Windows 10 compatible. That obviously does not mean you cannot run Windows 10 or bypass the requirements for Windows 11. Now this support page lists the systems that can be updated to TPM 2.0 and the 790 is not on the list. But to answer your question, no there is no way to update the system to have Secure Boot and TPM 2.0. and you cannot upgrade to a Gen 8 CPU which is the oldest generation that MS is supporting on Windows 11.

@bradthetechnut I am using the DT Model Dell OptiPlex

That's granted you don't have the USFF.

Just because Dell decided that the system was not Windows 10 compatible does not mean it would not run Windows 10. I have a Latitude E6420 that was also listed by Dell as not compatible with Windows 10 but it runs like a charm. Sorry if my statement mislead you.

As for Windows 11, it was not Dell's decision to list your system as incompatible with Windows 11. That was entirely a Microsoft decision. The system requirements have left a number of manufacturers having to remove Windows 11 support for their older systems not meeting the MS requirements. Now as for the MS statement. MS has threatened to remove support and updates for any unsupported system that has Windows 11 installed. As of this date that has not happened and it may not ever happen. But the possibility exists that one day MS will withdraw support and updates and may in fact block those systems from running Windows 11 in the future. It's MS world and we just live in it.

Yah, funny how MS listed minimum requirements for Win10 as 1.0ghz CPU and 1GB RAM.  Those will definitely run it slower.  4GB RAM is ok, but 8GB runs it faster, which you have.

@JOcean knows your 790 can run Win10; and Win11 with checks bypassed.  Just due to the age of those systems, Dell didn't validate the system for those OS'.

I had Win10 on my 755 and when we had Rockstar speedstep, he had a GX620 running Win10 stating that's as far back as it can go with an Optiplex.

I was so used to seeing some speedstep posts, I just remembered it might be false that Win10 can only go as far back as a GX620.

The laundromat here in town as 2 old PC's for public use - a 380 SFF and 760 or 780 SFF, both with Win10.  I tested them out a ways back and the 380 was herky-jerky just for the internet.

Dell probably doesn't care any more about system requirements for the Dell OptiPlex 790 since it is such an old model and they discontinued it. So that's probably why they say that the Dell OptiPlex 790 isn't compatible with Windows 10/11