first, did you check the md5 checksums of your downloaded iso ? if it doesn't match, you must download again.
to check the md5 sums,
if you have windows, have a look at this
if you have linux, see the "md5sum" program.
if your md5 are ok, try this :
did you try to boot with cd2 ? it a known bug for the cd1. if this way doesn't work, let's look :
copy the cd1 in a directory that would be not overwritten or formated. enter in the directory and go into Mandrake. you'll see RPMS, base, share...
eject the cd1 and put the cd2. go into Mandrake and you'll see RPMS2. copy RPMS2 in the same directory as RPMS (on the hard drive).
do the same thing with cd3.
now if you have a cd-rw, burn "boot.iso" which is in the "images" directory of the cd1 tree. i suggest you a cd-rw because the image is only 8mb ! it would be shame to burn a cdr for only 8mb
now reboot with this cd (contening boot.iso). select your installation mode (normal or expert), and let your computer... when linux asks you the installation method, specify him "Hard Disk", choose the partition where is the tree of the cd1. normally, it will load the graphical interface