If when you start your computer and the boot screen is presented and Windows is the first Operating System offered, while the Linux is offered in the Second position, you have a computer where Linux was the first Operating Stystem installed. If that is the case, Windows boot loader will not have been installed properly.
Windows is particular. It must be installed first to accept a dual-boot. It sounds like you already had Linux installed prior to installing Windows. That is why Linux works and Windows does not.
To fix the problem, you must start from scratch again. Format the drive you want to use and install the Windows OS first. Once installed, try it. I am sure it will boot properly.
Once Windows is installed, install the Linux application you want in the dual-boot configuration.
Now when you restart your computer you will have a screen open that offers choice between the two Operating Systems. The Linux application will be first on the list, and open automatically if you don't make a choice to open the Windows OS, but by selecting the Windows OS, it will open.
One other thing. I do not know the older computer you are making work again, but you also must check the Windows and Linux machine requirements to make sure that the Operating Systems you want installed fall within the capabilities of the older machine. I don't think this is your problem, but it may be.
I read your post carefully and glanced at all the replies. And if I have stated your situation accurately, the newly formatted and installed dual-boot should work just fine.

Of course, if I have misunderstood, my solution may not correct the problem. If that is the case, it sounds like the Windows application has been corrupted and probably needs to be re-installed. If that is the case and the repair option is not available, you will have to follow my instruction anyway and format the drive and install Windows first and Linux second. I am not sure which boot loader is the problem, but with Windows OS installed first, the boot problem should be solved.
