There are several ways to do so. In my opinion, the better and easiest way is using
BSPlayer. With this player, audio and subtitles can be selected with a simple menu (just select "open subtitle file..." and click the ".srt" or ".sub" or ".txt" etcetera file you want).
Or, if you prefer, you can use adiccional software added to your favourite player like VobSub, which will load any subtitle if the name of the subtitle file is exactly the name of the avi file (you'll need another exterior program in order to change subtitles position on the screen, color, or adjust timing, while BSPlayer allows to do it
on the fly).