Welcome to LQ, Paul.
I'm affraid it sounds as if you've got yourself some dead discs. If you had said that you had only tried it on one machine, then I'd have probably suggested that you try some other RAM modules, but since you've tried it on 3 machines...the likelihood of all three machines having duff memory is quite low.
If you've got the time/patience then Mandrake 8.2 is now available for download from: www.linux-mandrake.com
If you can, however, get in contact with the person that sold you the CDs and ask them about what their 'policy' is on replacing duff ones. I would have thought that since they probably got it from downloading it, they'd be quite happy to send you some replacements for the cost of a stamp...but better check first.
Hope this helps, and hope you get things sorted soon.