Does you computer have a restore partition on the hard drive? If so, you may be able to start the computer then press (I think) the F8 key and see some restore and repair options. Note that if you updated from a previous version of Windows to Windows 10 this option may restore the original OS. I don’t know if a Windows 10 update updates the restore partition. You can also visit the Dell support website where you may be able to download a restore image for free. You will have to provide your PC’s service tag number (which should be on a sticker on the CPU box) and some other information, but it may save you some time and money. Best of luck! Richard
You may have a short in your secondary winding. A short will still register as continuity, but the output side of the transformer will be effectively non-functional since it will be unable to convert the magnetic field from the primary winding into electric current. Unfortunately these can be difficult to troubleshoot. You may be able to find some information with a web search, but it tends to get into sensitive measurement, and honestly unless you're willing to re-wind the transformer coils you're probably going to spend a lot of time (and maybe money) for little gain.