this hasn't been a problem for windows prior to vista. but really get me irritated that on vista, pidgin (the former name Gaim) does not automatically apply the default Chinese font to its interface. this has to do with the gtk behavior changes in vista. you probably know how to deal with the situation by setting gtk-font-name to a Chinese font, but if you are like me, you will never be satisfied with the Chinese font's performance in English characters! i am writing this post hopefully which helps some of your simply annoyed souls too as i can't get anything related by google. (is it because i am the first Chinese user of both pidgin and vista at the same?! lol...)

put the following lines to %appdata%/Roaming/.purple/gtkrc-2.0:

style "purplerc_style" {
font_name = "Segoe UI,SimSun 9"
}widget_class "*" style "purplerc_style"

and here you are, the beautiful Chinese characters again!