Used to run dual win7 and 8 but I upgraded the 7 to a 10. Which has its ups and downs but at least I got a legit copy of windows now.
Also if you upgraded your windows, you should have a old windows located in your os drive which is old files and setting from your previous window. Mine was around 70gb so I just deleted it But am guessing you already know that.
I been 'hacked' before it can be scary and stressful times but you can easily solve your problems by going to a different PC and changing your email first then your other accounts. Starting with accounts that are linked to bank cards to items of value like a Amazon account or steam account. Ending with social accounts. After your done, return to the infected machine in which you can re install your operating system which would be best or finding the malicious file/folder. Most likely a key logger. However the program may just duplicate itself around your machine which then would be difficult to detect.
btw steam only returns stolen items ONCE so best protect yourself.
I study IT forensics and security. Currently entering my third and last year but I don't know much, am just a captain of a ship.