You're using LabVIEW 7 or 7.1? If you're using 7.0 and somehow your VI is pointing to a version 7.1 function, right click on Read Characters From File, select Replace, and select the function that's on the File I/O palette. If you managed to somehow save a version 7.1 VI in the same location as the 7.0 function, you could reinstall LabVIEW or try doing a repair. If you close out LabVIEW completely, restart it and place the function on your block diagram and don't get the error, doing the replace will work.