AVISub is a software that lets you merge subtitle files (SRT, SUB, TXT or VOBSUBS) with AVI video files. What it does is merge the files generating a new AVI file.
You can specify the size and font of the caption, as well as its position, borders, and so on.
Because it is an old tool, its use is not so indicated, since there are several more modern tools that are more apt to perform this type of procedure.