Optiplex 990, request service for out of warranty?

The only way to run Windows 11 on that system is in unsupported mode -- it does not have a CPU that Windows 11 supports and cannot be upgraded to meet the requirements.  Neither will Dell have support for a system this old.

It's time for a new(er) system.

If you want to run a supported OS, there are many Linux distributions that will work.

The diagnostic code of 2000-0142 is a memory failure. Should you wish to continue using the system, I would look into replacing the RAM. However, as @ejn63 said, you may want to look into a slightly newer system for Windows 11. This model is end of life so our out of warranty teams would likely be unable to assist much further and we wouldn't have any stock for its parts. 

2000-0142 is not a memory error - it is a fail(ing or ed) hard drive.

Look for an 11th generation or newer OptiPlex -- these start at about $250 used.  That should be good for a long run with Windows 11 (there is evidence Microsoft will in the near future, raise the CPU floor for Windows 11 from the 8th to the 11th generation CPU).

These would be the OptiPlex 5090 and newer models.

@ejn63​ ah yup, got my codes confused! Good catch, 0122 is the memory failure. 0142 is indeed a failed drive

@ejn63​ I didn't say it was a memory error - when I run chkdsk it shows no error after the diagnostic error. I'm going to be forced to buy a Dell Desktop with Windows 11 loaded on it close to October, 2026.  

@DELL-Daniel V​ Thanks for your help

@ejn63​ Thanks for your help!