How to handle the Back and Cancel buttons in custom page

How to handle the Back and Cancel buttons in custom page
Hi All,
This is Praveen Nukasani.I have written a Custom page for may Installer ,i have written that page as 3rd window for my Installer. But im not able to handle Back & Cancel buttons in that page.
In that window,I created a browse(FileRequest) i,.e my license file. After that, i am able to read the content and validation is Successful(I have done that process).
But My Problems are:
1) If I Click on the Next,I need to validate the License File, If Validation is successful go to another step, Else need to display error message and Abort the functionality. i,.e Need to display the 3rd window only. But What i am getting is, If validation is success or failure which is going to another(4th) window even though, I used Abort Command after checking the validation.
2) I am not able to handle the Back and Cancel buttons. In custom page, If i click on Cancel and Back buttons, Those(Buttons) are validating the license file If we browse License file in file request or Not.
Please check for my requirement. I hope will get a response.

Regards
Praveen