With instant messaging, you have to do it like that since if you are logged in from multiple places, the system wouldn't know where to send you your messages... so that's why Yahoo IM, ICQ, AIM, etc have to enforce one active login per account at a time.
But with a forum like this, all you do is post with your account which can be done from any PC. Now obviously you can't log into the chat function from different PCs ... although I haven't tried that yet!!!!