Rundll32.exe and Tapi.dll errors

When dialing out to the net I get a this error “Rundll32.exe has performed an illegal operation….”. I check the next bit and it is caused by the Tapi.dll.
I have replaced these files and still happens.
Uninstalled the modem and dialup adapter, communications and still no luck.
Can some one please help…As I don’t want to reinstall my machine.
Thanks