Please back up all your grades to disk and then delete the c:\1STCLASS folder and then reinstall. You will loose your settings (eg. Name, Password etc.) but these can easily be entered again.
VBRUN300.DLL is the run-time DLL that must be found on your system for any Visual Basic 3.0 program to run, 1st Class Gradebook needs Visual Basic to run. Normally you find this DLL in your Windows/System directory.

If you are on a network and your users get this error you will need to copy the VBRUN300.DLL from your server to your Windows/System directory.