Integrate SharePoint Login With ASP.NET Web Page

Hi, this is really urgent... if someone can help on this it will be great... My
requirements are:

 I have a seperate ASP.NET application on which I have a login page which asks
users for username and password. I validate this against Active Directory. If
the info entered by user is correct, I want to redirect them to SharePoint
website, but I do not want SharePoint to pop up the ugly Username and Password
dialog box again. I want to programatically log the users into sharepoint before
i redirect them to sharepoint to avoid the ugly dialog box.

 I have tried everything but have not succeeded so far. I referenced the
LoginWindows web service provided by sharepoint but it throws a soap exception.
I also tried to use System.Net.HTTPWebRequest object to send a request to the
sharepoint web page with the username and password. the request works fine
without any error but the moment i do response.redirect after the http request,
sharepoint shows the username password dialog box again (even though just before
redirecting, i used httpwebrequest object with proper credentials to get a
response)

Any help would be greatly appreciated