that old windows 7 iso is official from Microsoft or kind of modified? always download windows iso only from microsoft website, because any other websites can really put bad files inside it (or if it's a modified iso with a keygen activator, then that's it)
same thing for the games: a mod or a plugin for a game can be a false positive. for pirated/modified games, yes there can be malwares, but most of times is a false flag.
regarding the passwords, I recommend to use another pc and change your main passwords (email, bank account, paypal, etc..) and enable sms code or a strong login. for example, my bank requires secret password from a certified phone to do most of the things.
and for the antivirus, really you shouldn't trust windows defender. is better than any free antivirus, but is way under any paid ones, like i said Norton antivirus. you should install the 30days trial of Norton and do a full scan (can take hours, so I recommend to do it during the night). If Norton doesn't find anything, you're good, and if it finds something, it can fix.
I know it can be stressful, but see this as a good opportunity to organize better all your software licenses (print them on a paper, save them somewhere else) and install a fresh copy of windows and you'll have a better computer ^-^