Are you sure the discs aren't corrupted? You can check using MD5sums. Look at
this FTP dir (it's an
example!). It has files in it, file.iso, and it has file.iso.md5 in it. You'll need an MD5 program to check the MD5 of the ISO you downloaded, then compare it with the one in the *.md5 file. You could also try to get CDs burned by someone else (
Ubuntu ships them for free*).
This is a great help:
How To Check MD5sums On A Linux Iso Image (LinuxISO.org)
*Though these aren't Mandriva (they changed their name from mandrake) CDs, ofcourse
