Windows 7: "Missing Operating System"

Hello all.

I intend to dual boot Linux Mint and Windows 7 on my very old MacBook Pro 4.1 (Early 2008) and it's been a journey.

I managed to get Linux to work (overriding Grub bootloader in config plist file), but I can no longer get into Windows 7 via Opencore, it shows the message "Missing Operating System". So I followed this guide and I successfully installed rEFInd and got it to show Windows on their boot menu. Thing is that now it just shows a blank screen.

I REALLY don't want to reinstall Windows again. What should I do?

Thanks in advance for your answer!