Is there a setting to force Adium to open a new chat in a new window? For Adium, a single window holds many chats.

from official link (http://trac.adium.im/wiki/TabbedMessaging) 
"A single window might contain 10 different tabbed chats"

Is there any setting where I can force Adium to open new chat in new window?
Yes. Under Preferences -> General, uncheck the box called 'Create new chats in tabs' as on the screenshot below.