IFrame security with parent same domain?

[Jéjé]

Hi,

I have a web page with an IFrame, this IFrame could display the content of a
web site on the same domain.

Parent page:
http://www.mydomain.com
IFrame SRC:
http://reports.mydomain.com

How can I prevent any warning to my end users?
The 2 websites are protected with clear/text authentication, can I prevent
any popup login window when I call a secured page in the IFrame?

I control only the web page which generates the HTML content with the
"IFrame" tag.
Its an ASPX web page.
So I can add something in my code.

Does using standard Frames instead-of IFrame can help me?

Thanks for your help and guides.

Jerome.