First combine your 2 CDs into one AVI. Rename your AVI and the IDX and SUB files to the same name. Then play the AVI in a payer that supports IDX/SUB and see if they line up properly. I have ffdshow and Vobsub installed, the subs show in both MediaPlayerClassic and MediaPlayer 11 for me.
I've got a feeling if you look harder you'll find SRT subs out there.
Also as Whappo noted you can convert IDX/SUB to SRT with VobSub-> SubResync.
ConvertXtoDVD handles both types for DVD creation.