I have one index.php file that is on abc.com server.
Here in index file i have frameset and src of that is : 'http://www.xyz.com/login.php'
All my php files are on xyz.com.

What i am having problem is when user open url : abc.com
And they get login page.
they are not able to login from here only in IE8, in mozila it works fine.

I have searched that in IE8 if subdomains are different then session will be losted.
So, now i got problem but dont know how to solve it.

Please help me...

I have fixed this from below help :
workaround when using session variables in a .php file referred by a frame (.html, or other file type) at a different server than the one serving the .php:

Place this header on the .php file that will create/update the session variables you want:

<?php header('P3P: CP="CAO PSA OUR"'); ?>
Texts from http://us.php.net/function.session-start