install an on-demand scanner or completely reformat my PC
I agree with what you said in your post
(the below isn't aimed at you, but just another post haha, saw the oppertunity to escape from under my rock and shine some light on some topics quickly):
I recommend to just reformat the PC and use an old, clean backup, since once your system is infected you'll never know if it's 100% clean or not even after scanning with every single scanner out there. For example, if you become infected with a rootkit which is brand new, how do you know it is really gone after using an on-demand scanner which detected some objects? You won't know unless you had previously analysed the sample to understand exactly how it worked and the changes it made.
As well as this, reformatting is a much better option than just reinstalling the OS, since malware can directly write to the HDD and cause it to come back after an OS reinstall (but this will not happen in the case of a complete reformat). That being said, some really rare malware may infect even the firmware (BIOS) to come back (for example), and then in this case you'd need to flash the BIOS... Or a hardware infection, replace the hardware components. Now tell me if Malwarebytes Anti-Malware will detect something like this? It won't.
Better be safe than sorry, just don't let the system become compromised and there won't be a problem.