usually Windows XP is very slick with Modem and Modem drivers. May be you issue is not the Modem Driver !!! may be you have resource issue !!! Windows XP and Windows 98 handle the computer resources differently, go to My Computer Property / Hard Ware / Device Manager / Modem /Property / resources ,, do you see any conflict in resource setting.
Another thing you could do if you have PS2 or USB mouse. Go to BIOS and disable both COM ports and see how your modem behaves on reboot.