An iso can be viewed with VLC player which can display subs, but I don't think that's going to work for you. An iso is an image file that's meant to be burned to a cdr/dvdr and if it has a menu, that will mess up the timing of the subs. You can use programs like WinRAR, Ultra ISO, DVD Decryptor, or DVD Shrink to extract the contents of the iso and rename the appropriate files to play with the .srt. For example, if the iso is a big dvd movie, you can open it with DVD Shrink, choose Re-author, and keep only the main movie (without menus). If the movie is over 1 gig, go to Edit->Preferences->Output Files and uncheck "Split VOB files into 1gb chunks". Hit "Backup" and the folder you save it to will have one big uninterrupted .vob file that you can rename to use with the .srt.
(Oh, and before "Backup" make sure to select "no compression" under Compression Settings since you only want to view it and not shrink it)