Override FormsAuthentication functions

How can I do this?  I'm using my own database, so I want to do my own authentication but still use:

if(FormsAuthentication.Authenticate(txtUsername.Text, txtPassword.Text))
FormsAuthentication.RedirectFromLoginPage(txtUsername.Text, true);
else
Response.Redirect("error.aspx"); If anyone knows how to override User.Identity.IsAuthenticated, that would be helpful too.I do have my own custom Membership provider if that helps.
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