If it's booting the OS that was on it when you found it, there's every chance the software you need is already installed. Mostly extensions and control panels.
There used to be a great FAQ on the AV Quadras by one James Wang, but all I can find right now is an archive.org copy of the index to his site. Poke around there and see what you can find.
http://web.archive.org/web/20090826131422/http://people.ischool.berkeley.edu/~jwang/pitstop/
/ETA/ helf found it:
http://rsb.info.nih.gov/nih-image/download/documents/av-faq-156.rtf (text file download)
http://rsb.info.nih.gov/nih-image/download/documents/av_digitizer.txt
http://rsb.info.nih.gov/nih-image/download/documents/quadra_video.txt