New motherboard typically means you need to reinstall the OS.
Since you appear to want to run Windows 7, you will need a legitimate license key so you can download the ISO from Microsoft
Windows 7 ISO Direct from Microsoft
However, this is just the ISO download. You would still need to burn the image to a device in order to use it as installation media.
If you do not have a legitimate Windows 7 product key, then I would suggest switching to Windows 10 and use their available program for creating the installation media:
Windows 10 Media Creation Tool
I don't think you will be able to do this at a library computer, however (check with the librarian) as this does require installing a program on the computer.
-Wolf sends