Sessions on NT

Hiya, Sorry for the newbie question but I'm having trouble getting session management going on NT. Configuration is NT Server 4.0 , PHP 4.0 (using the isapi dll), IIS. It seems that whenever I call the session_start function, nothing more after the function call is returned to the browser. If I edit the php.ini file to automatically start sessions no php pages are sent to the browser at all (although normal .html files are as expected) If I remove all sessions from the PHP pages, all is is well. I suspect that it has something to do with NT permissions and/or temp directory locations for the session files but I could be way off base. Does anyone have any ideas?? TIA, Matt