Most pci winmodems default to a certain port. So the old modem has been physically removed, right? That's step one.
Okay, boot to safe mode, go to add/remove programs in the control panel. Uninstall any modem drivers listed here.
Still in safe mode, open device manager, expand modems, remove anything left behind. Expand Ports, and make sure there is only one COM1, and one COM2. If you have duplicates, remove them all. Reboot.
If you had duplicate COM ports, Windows should reinstall them. If not, then go ahead and reinstall your modem device driver. Take whatever COM port it gives you.
Go to Control Panel, Modems. Highlight the modems, hit diagnostics. Select the COM port the modem is on, and hit more info. Reboot a couple of times, repeat. If this works, port should be okay. Now try to dial out.
If you still have problems, let me know.
Matt J.
Please always take the time to backup any and all data before performing any actions suggested for ANY problem, regardless of how minor a change it might seem. Also test the backup to make sure it is intact.