Change page’s titles – how to?

Hi

I’m trying to change each page title into something else. For example, each post page is named like: blog_name:post_title. Same with categories, archives, etc.

All I want is to change that title into post_title or something like that. Can anyone tell me what line in the code do that? I’m using default template on wordpress 1.5 and 2.0

Thanks a bunch,

Chris