I see you want to try to fix this while changing as little as possible (and buying nothing additional). Try these steps. It would help a great deal if you could back her geneology files up. You should get a Windows 98 boot disk if you don't have one already. You may also have to get the dos 'deltree' command put on your book disk (as I'm not certain that the Win98 boot has it).
1) Try booting up with the boot disk. Click that you want to boot without floppy driver support. When you get to the dos prompt, run scandisk. If it reports no errors, or if it claims to have fixed them, then try booting as normal.
2) If you need to try to reinstall Windows, because it's still not booting into the operating system, then boot up with the floppy (no cd support) and delete the windows directory (as, say, 'detree c:/windows'). Reboot when this is finished.
3) Get the Win98 cd and make sure you have a correct product ID code. You may need to go into your bios settings and turn off cmos antivirus detection, because this can prevent the installation from proceeding. Boot up with the floppy, but this time choose that you want cd rom support. If your windows directory has been deleted, and the system now successfully reads the win98 cd in the cdrom drive, you're ready to install win98 as usual. Go ahead and do that.
If at any point you can't boot to the dos prompt, even using the boot floppy, or if the installation does not ever fully proceed to completion, or you simply can't get windows 98 to run because you continue to experience the 'overflow' error, then you likely have a hardware problem, and that may require taking the box into the shop. If at any point you become anxious that you may accidentally delete all your mom's geneology data, then you might again consider a trip to the shop.
Good luck!