Windows 10 not accepting a Pin, Wants a Password

Hello, Everyone.
Recently I bought a new PC for my sister on her birthday. I was trying to speed it up before installing a new game as it runs really slowly, with the CPU at 100% when doing nothing at all really. I’d already disabled all the startup items, and found an article suggesting I use MSCONFIG to disable all the Microsoft items.
Now when I have restarted the computer, it’s asking for a password (which we don’t know) and not the usual PIN. I’ve tried the Microsoft password reset, but the computer cannot connect to the internet so I don’t think this works. The forgotten my password link on the login screen doesn’t work. There are no options to change from a password back to a pin. There is only 1 login, no administrator option. The BIOS options are unfamiliar.
Does anybody have any suggestions? Thanks.