As I mentioned in my OP there is no boot.ini file. If there is another way to check if I have dual boot configuration I'd be glad to do it that way, although in my knowledge no there hasn't been a second OS installed/configured in the drive and there is no second hard drive or any partition that another OS could have been installed.
Regarding the computer's firmware, I tried different methods (How to Know If Your Computer Uses UEFI or BIOS | iSumsoft ) I found online to check it:
I checked on the Run->msinfo32
On the C:\Windows\Panthers\setupact.log file, but there was no entry if it is UEFI or BIOS.
And I also checked on the Disk Management window:
I believe it is BIOS and not EFI/UEFI.
I had disabled the antivirus protection before starting the conversion, yes. If you think that the antivirus might be the issue, I could try uninstalling it and have a go converting it with no antivirus at all.
Let me know if there is anything else I can check or if I should try uninstalling the antivirus and trying again to convert the laptop.
Thank you!