The installation can go no further if you can't buy a genuine XP Product Key so you may as well pull the CD and restart the machine. If you need the BIOS settings and don't know the password, you're in a mess. You could take it to a professional and prove ownership and see if they can help you but I doubt it would work.
Your best bet is to stay away from BIOS and go to work on your Windows 7 system. Upgrading to Windows 10 while you can still get it free, might solve a few problems. Anyway, it's better to move forward than back to XP.