Problems with sessions

Hi I've developed a Download Manager software for my company which run as a PHP-script with a control panel and stuff. I've got a MySQL DB with the username/password and home directory. When a user logs in it checks the username and password, if correct, create a session. The session-file is created and the information inside it seems to be OK. But when I try to open the session again to read it, it returns nothing (NULL). What could be wrong. The computer I developed the software with is a: Windows 2000 Professional OmniHTTPd (latest build) PHP 4.0.6 MySQL (latest build) The server which the application is supposed to be on uses: Windows 2000 Professional Server IIS 5.0 PHP 4.1.0 MySQL (latest build) How come it works ok on one computer and not on the other. I've tried to use session_save_path, but it doesn't work. And how come I can't use $DOCUMENT_ROOT anymore? I get an errormessage of it on the server. Thanks in advance.