Desperate Help needed

Hi, I made a database chatroom that seemed to be working perfectly fine - now, it seems, if more than two people log in, the 1st person of the three automatically gets logged out - its because i tried to make an automatic log off for those who cross the window.... How do you detect if a person has closed the window (hence killing the cookie) without logging off (so the database still has him listed, and therefore is shown to be online)? Here's the scenario: 1. one table lists the chatters' names 2. second table has 25 rows for each messages, they keep getting replaced as the new messages come in the problem: the chatters table lists those users who logged out ages ago... thanks.