[CURRENT ISSUE] Sims 4 already running

@tiffy_is_here  Don't worry about the Antimalware service; it's part of Windows and shouldn't have any effect on Sims 4.  I don't know exactly what should or shouldn't still be running when Defender is disabled, but if this service were a problem in general, we'd see a lot more reports.

I totally understand not wanting to reinstall all that data.  I do want to mention that this is one reason some people partition a larger drive and install Windows onto a 100-150 GB partition—you could wipe just that part and keep everything else intact.  I promise this isn't a lecture or even a suggestion—I haven't this myself and don't think it's strictly necessary.  But it's worth keeping in mind, perhaps for your next computer, if you tend to run into issues that a clean Windows install should solve.

For this issue, right now, please let me know the current status of the game in a new admin Windows account: how far you can get in trying to play in a clean user folder, what error you see (if any), etc.; and whether this changes when your computer is offline.  Since you've seen a few different errors, I want to avoid any assumptions.

Please also look for a current Sims 4 crash dump.  There might not be any, depending on whether the game is currently crashing or simply being blocked from starting, but it's worth looking.  Click Windows key-R and copy and paste this:

%LocalAppData%\CrashDumps

When you enter, you'll see a list of .dmp files.  If one of them is from Sims 4 or the EA App, please upload it to a third-party filehosting site (Google Drive, Dropbox, etc.) and link it here.  If you see a crash dump that was written at exactly the most recent time you tried to play, I'd like that too.  But you can ignore older files.