or the simple way is just keep a flag in database means when the user logs in set the flag to true and check this flag's value on Page Load event of the login page i.e if the flag is true the it will automatically redirect the page to another page else will not do any thing means will simply load the login page. and set flag to false when logged out this will serve your purpose. In this case you can open no of windows dont worry it will definately open as logged in.
Darshana Pathak