OK, this error can arise due to a number of issues.
1. The DVD that you bought was burned incorrectly or is not compatible with your DVD drive. Is this a full commercial version of Mandriva, or a DVD burned by someone who is selling them online? If the DVD was burned on a DVD disk that is not compatible with your particular drive, this can explain the problem also. I would never buy a home burned DVD or CD disk from someone, as you never know what you are getting.
2. A hardware problem between Mandriva and your system. Can you post what make and model system this is, or what motherboard that you are using, if this is a self-built system? This is a 64 bit version of Mandriva that you are using, so I assume that your system is an Athlon 64 bit motherboard and processor?
In some systems, say with sata drives (as many Athlon 64 bit motherboards are), you may have to change a setting in the bios to get Mandriva to see the CDROM drive correctly.
3. Is the DVD drive an internal drive or a USB DVD drive? I have an external DVD drive that is a real pain to boot from on my motherboard. If the DVD drive is an internal drive, is it attached correctly to an IDE (not RAID) port on the motherboard? This would apply to a self built system, or is you installed the DVD drive yourself (it was not installed by the manufacturer).
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"I love it when a plan comes together." - Hannibal Smith