@sebastian To be honest, I don't quite understand what you're saying, but if you're afraid of installing Windows on top and it erasing important data, like photos, I don't know if you have the option to take out the hard drive, plug it into a USB enclosure and use it as an external drive. That way you could use it as if it were an external drive, or a USB stick. You could copy the data to another computer and then try to repair Windows.

If you insert the Windows DVD or USB, there is an option to repair Windows. It may take a while, and depending on the version of Windows you have, it may vary a bit, but in general it is a simple process that, as I say, may take a while, depending on whether the hard drive is mechanical or SSD.

If you tell us the version of Windows, we'd better.

Best regards.