"The buffer is invalid."

Again, another modem error...

If you have installed any new software doublecheck that it didn't modify any modem settings on its own..

You could try uninstalling and reinstalling the modem as per manufacturer instructions.

You could also uninstall and reinstall dial up networking.

Under modem properties, (make sure the correct modem appears in the drop down box, and it's not a 'standard modem' or some modem that isn't yours), check the port speed, if you have a 33.6/28.8 modem, make sure the speed is no less then 38400, 56k modem, nothing less then 57600 (38400 is acceptable under certain circumstances here if you have a poor quality line and are using the 56 modem at 33.6 instead).

For more advanced users go to 'Port Settings' and check to make sure the FIFO buffer settings aren't set to low.. Again, this is for advanced users...