It took me the long afternoon but I didn't find the clue how I can redirect from the Login.ascx.vb.cmdLogin_Click method to the signin.ascx.vb object (I made a breakpoint at the Page_Load). When I traced the cmdLogin_Click to the passage:
....  
' portal tab
If PortalSettings.HomeTabId <> -1 Then
     Response.Redirect(NavigateURL(PortalSettings.HomeTabId, "Login", "returnurl=" & ReturnUrl), True)
Else
     Response.Redirect(NavigateURL(PortalSettings.ActiveTab.TabID, "Login", "returnurl=" & ReturnUrl), True)
End If
....
so far I understood here must it happen but how ???
From where does I take the /admin/security/signin.ascx ???
The cause: I'm finding (to code) a solution for customized skins for each portal on one dnn installation. I want not to touch the original login/logoff and signin files regarding the future updates.

Please can anyone help me with my problem or give some hint to solve it ? 
Thanks a lot in advance.

Thanks a lot it was a real help for me - I understand now how it works and my prototype is finished - when develop new administrative controls I'll do it over the NavigateURL method and the table in the database.

greetings 
guy  

To make a page the first page of your site, you could specify it as the Splash Page in your site settings (under Advanced Settings --> Page Management).  So, all you should need to do is to create a page with the Account Login module on it, and then make that be your Splash Page.  Then all traffic to your site's root will be redirected to that page.

Hope it helps,