Fatal error: Class 'Model' not found After Upgrade to version 2.0

[eluser]WanWizard[/eluser]
The first error is from trying to load a library the wrong way, so check your controller.
The second is because of the first.

The validation library is deprecated since 1.7.0, and should not have been used anymore in 1.7.2. It is removed as of 2.0, you should now use the form_validation library.