I'm sorry to be bearer of bad news there is a good chance you either
1. have bad ram
2. CDROM is on a Bad IDE channel or is itself had it.
1st if your lucky it's the cdrom being a bitch find another one and plug into a different IDE channel on you motherboard.
but if you can't fix that / there's no spare parts
What version of Mandrake 10.1 you have?
if it is Community
look for the Official version
But many believe that Slackware is the fastest for old computers, but the installer is a fair bit more difficult then the easy wizard mandrake provides.