I am afraid I cannot come up with a slick solution for you. It seems fairly certain that you corrupted either a file, a Registry entry for TAPI, or the modem interface. I am interested in what happens if you put a plug-and-play external modem on the serial port and restart. If this modem also fails to work, then you may be stuck doing a reinstall.
You might want to try the Last Known Good Configuration option to recover a previous Control Set. Press F8 at boot time and select the option. This would restore portions of the previous SYSTEM hive in the Registry. If you corrupted the Registry, then this might do the trick.
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