I used to have a problem of subtitles. The names of the subtitle files created by VobSub are the same as the avi, but I still can't see the subtitles. And after that, I realised that it was because I had uninstalled VobSub.
Quoted from doom9.net
"DirectVobSub is simply a DirectShow filter which can be used to display subtitle files created by VobSub during playback in Windows Media Player, PowerDivX NextGen and maybe other players as well"

So, I reinstall VobSub again and the subtitles options are now avaliable. Yeah;)