One thing to add is if your mom is taking PC classes, the instructors assume that their students have Windows boxes at home.
My mom's first class was an intro to PCs, using Win9x boxes, and then her second class was for MS Excel. She would be less than pleased if I installed Linux/StarOffice, since she would then have to adapt what she already learned to the new environment.
The other thing is that with having Win98 on her box, I don't have to give her any lessons, as she gets OS and Microsoft app-specific instructions in class. (Besides I'm 50 miles away and only get to visit 2x per month).
That last thing is that I support Win9x/NT/2k full-time at the office, and only play with Linux as time allows, so my Linux support experience isn't as deep such that I could rattle off answers to common questions. So having Win98 on my mom's PC is what the doctor ordered in my case, though if the OS ever broke, I would migrate her to Win2k for the stability.