Here is a simple way to redirect the New Item button in a Sharepoint list.
Maybe you need to redirect this button to your own ASPX page in the /_Layouts/1033/ folder.
Go to the folder of your Site Definition on the Sharepoint server, this is usually located at c:\program files\common files\Microsoft shared\web server extensions\60\Template\1033\\Lists\
There are usually at least five files in this folder:
AllItems.aspx for viewing all the items in a list NewForm.aspx for creating list items DisplayForm.aspx for displaying list items EditForm.aspx for editing list items SCHEMA.XML CAML file describing the list.
Open the NewForm.ASPX file in your favorite editor and replace the content with the following lines:
--- cut here----
<!-- _lcid="1033" _version="11.0.5510" _dal="1" -->
<%@ Page language="C#" Inherits="Microsoft.SharePoint.WebPartPages.WebPartPage,Microsoft.SharePoint,Version=11.0.0.0,Culture=neutral,PublicKeyToken=71e9bce111e9429c" %>
<!-- this line redirects to your ASPX file -->
<% Response.Redirect("../../_layouts/1033/Myfile.aspx"); %>
--- end cut here ---
That's it. The New Item button for this list defintion will now redirect to your ASPX application in the _Layouts folder.
For more information on Sharepoint List and Site Definitions:
Customizing List Item Forms in Windows SharePoint Services
Creating a List Definition