I think most of the .srt softsubs will play by default…
However, if you have softsubs in other formats, you may need the right plug-ins or codecs.
I use Advanced SubStation Alpha format (.ass) a lot… and to view this, you need to install Vobsub.
Or, you may wish to install the CCCP Codec Pack which will work too.
Put the video file and the softsub file in the same folder in the computer.. and rename them so that they have the same name with just different extensions.
When you play the .avi file with a player like Windows media player, the subtitles should show up automatically.