Since your MB/bios is so old it may not have the "plug n play aware os" feature. You could always try and update the bios.
If you don't know what make/model your mother is there should be some lettering/numbers on the board somewhere. Type those and motherboard into
www.google.com and see who else has asked the same question.
I usually get a pointer to the manufacturer and hit their web site for a BIOS update.
Is the com port an on-board one? If so then I would try changing the settings to COM2. Do you have anything else connected to any COM ports? mouse maybe?
COM1 and COM3 share some resources (can't remember which, IRQ maybe?) so you might be getting a conflict that linux can't work around but windows can.
Besides that, what do you mean 'linux never detects it'. It is my understanding that linux doen't detect it, you have to tell it that the modem is connected to ttyS2.