Try accessing the index directly. Such as:
http://localhost/osticketdir/index.php(http://localhost/osticketdir/index.php)
If that works you may not have set index.php set as a default document.
For IIS this would be:
IIS Manager -> site name -> Default Document
For Apache edit your config file and look for:
DirectoryIndex
ensure that index.php is listed.
Note: this should also work for Apache2