Class 'CI_Formgenerator' not found in C:\Users\mycode\system\core\C

There are four ways that this can happen:

- You've made modifications to system/core/
- You're overriding one of the framework's core libraries (which is why I asked you to tell what you have in application/core/)
- You're using a third-party plugin that does one of the above two conditions, but you're not telling us about it
- You're manually calling load_class('formgenerator') somewhere in your code

Truth is stranger than your expectation. :-)

In this case, a controller class was loaded as a model:
$this->load->model('register');

Quite stranger indeed ... yet makes a lot of sense when you see it (2 violations of the file-class naming convention + a name collision).

Btw, this can only happen on case-insensitive file systems.