A very easy program to sync either .smi .srt and/or .sub format subtitles is Vladimir's SubSync and I use it rather frequently (as indeed it happens moreoften that the sources of the avi videos and of the (mostly) .srt subtitles differ, than that it occurs that both originate from the same source).
You can find it here:
http://users.pandora.be/vlad/subsync/index.htm