I have a project that is currently all over the place and i'm thinking of making it MVC.
The problem is that the system is currently being used and I can not change the interface (It's in frames :s) Also there are certain things that I will need to handle myself such as password generation, login and user levels.
I already have the model side down of the MVC so I am wondering is it worth using a framework like Zend Framework or CakePHP or just to code my own View and Controllers to work around this problem?
I am going to have to work this in slowly and I'm not sure if i will be able to do that if I use one of the ready made frameworks.
Wheel reinvention is bad. Use a tried and tested framework like Zend Framework unless you have a really, really special reason not to.
Not knowing the details of your situation, I'd say that the fact that the system is currently running would make a general purpose MVC framework difficult to integrate.
Having said that, many MVC frameworks are composed of modular components that can be used as stand-alone entities. You may be able to pull off certain features of those frameworks for use in your project.
User authentication, session management, and password handling is one area in particular that should only be built from scratch if you have a great deal of experience in that area.