motorola c350 modem detection

i have pIV 1.8Ghz with 256mb of ram. i have windows xp & windows 98se installed on my pc.i want to use a dial up connection using my mobile thru gprs. my problem is that i am unable to detect the motorola usb modem in the widows 98se.on same pc in windows xp the modem is detected on com3 port. i tried to manually install the modem, but it didn't work. what should i do?plz help me.