Post 4 is correct, and failing that, you could likely buy reinstall software from Dell for that PC.
Of course, we should make such from the image partition which is usually provided rather than the DVD installation media. By which you can usually reinstall the OD. Did you try that option (F10 during POST)? .
I probably did, but it has been a while since I did that. I was able to get into the mode to put it in original factory software condition, but it locked up at some point in the process. Also had issues with the USB mouse at the time.
My son was in the hospital from Nov 26-Jan 2. He had respiratory failure and was on a ventilator for 3 weeks. Then he was in physical rehab for 10 days, before he demanded we bring him home. I attempted the computer restore after my malware cleanup failed. It was probably some time back in February. It has been only recently that he has been able to walk some without the walker. He has not recovered the hand dexterity to operate a keyboard. Limited feeling in his fingers and toes. But he is healing. He is almost 40, had pneumonia, plural effusion, mild heart attack, bacterial infection in his blood and terrible edema in hands, legs & feet. But we still have him.