Run REGEDIT and check the registry for this key: HKEY_LOCAL_MACHINE\Enum\PCI\VEN_12B9&DEV_1008\xxxxxx\PORTNAME (xxxx's are the mem address for the PCI slot. This key may vary, and is for a 3COM PCI hardware modem). If you have that key, double click the portname and change com5 to whatever com you need, probably com3. Then expand HKEY_LOCAL_MACHINE, System, CurrentControlSet, Control. Hightlight the CommAlias and click edit. Then click delete and confirm deletion. Reboot.
***Registry editing should be done only after backing the registry up. It can cause nasty things like no loading the OS and causing the OS to have to be reinstalled. Please take precations!***
Let me know if this helps.