Navigation button disabling and page skipping

I am trying to do 2 things, and i simply cannot get the info i need on how to accomplish it.

Firstly - I need to disable the NEXT button on a custom form until the user has completed the username/password boxes.

Secondly - I need to be able to show different pages based on the result of a function I have written.

Can someone point me to some samples that will show me how to do this?

TIA

The "notify" feature of InstallOptions allows you to disable the button if the user/pass are not yet given. If you don't want to show a page depending on previous input, you can use Abort in the pre-function.

Great! Thanks! Now where do I find that in the manual?