Header's Combination?

[nhatrang]

I have forums.domain.com and gallery.domain.com and they are all intergrated... however, when i install another software that has the login component, which is not related to Vbulletin, i get this message

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/domain/public_html/program

Anybody knows what's going on? How can I clear the session cookie I guess..