***** ViPER Guestbook is no longer supported by its developers and is not recommended for use. This is why it is no longer available in the Fantastico autoinstaller******
Instead, you can use the Ardguest guestbook application.
If you are getting Internal Server Error message after you have installed ViPER Guestbook via Fantastico, the first thing you should do, is to check what is the exact error that you're receiving in your cPanel -> Error Log.
If you see an error like "/path/to/your/root/" is writable by "others" this should mean that the permissions are 777. Please set them to 755, since this is the maximum allowed value on our servers. You can change the permissions from:
cPanel -> File Manager
When you find the file or directory, which needs to have its permissions changed, you have to click on its name and choose "Change Permissions" from the upper right corner of your screen.
If the permissions are OK, please make sure to check the .htaccess file for some permanent redirections or other rules(you'll see if there is an error with the .htaccess again in the cPanel -> Error Log), which may cause this error. In this case you should delete or at least comment those lines in the .htaccess file.