Either your optical media (CD or DVD) was burnt incorrectly, or the media itself is damaged, or your optical drive has some problem to read the media, or you downloaded the wrong ISO.
For any of such problems, there is already enough info published, whether in the forum, or in the main website (FAQs, tutorials, troubleshooting...) or in the wiki.
Having said that, if what you want is to retrieve the important files first, I would suggest using some other
(which is the main website of this specific project).
You could try with some free security LIVE CD / UFD (AVG, AVIRA, others) or some other type of tool that would be more comfortable for such task (for example, PartedMagic is included in UBCD, but it can also be used separately). I would suggest to try with security - related tools first.