The CD could quite possibly screwed, I dont run mandrake personally, but I know red-hat has a mediacheck feature on there CD's which checks and MD5 checksum.
Mandrake may have a similar feature, or if you got someone to checksum there cd you could do it manually and see if they match.
You do an MD5 sum by doing this...
$ md5sum /mnt/cdrom
You can do it to other files to, get someone else with mandrake to do this and tell you the checksum.
If your numbers match then the problem is somthing else, if they dont, then you will need a new disc im afraid.
You could try cleaning it as well, CD's tend to get dirty, but some dishwashing detergent and a soft cloth can clean them. I do it often to CD's that have sat on my desk and gathered much.
Good Luck.
