The CD release is 700MB and there can be LiveCD's or even the first, second, third, and so on disks for the set. Or you have to download the 4GB DVD for Mandriva 2008.
I also had the same problem with Fedora 8 on my laptop. It booted, and then said it couldn't access the CD-ROM for installing the media, and asked me if I wanted to do an install from the internet, ftp, http or whatever. I had another machine that had Linux on it already, and put the DVD on that machine and then did a network install from that mounting the DVD to an ftp server

I'd download Mandriva 2008 and see, you might end up with the same problem or you might not. I'd suggest you download disk 1, and see what happens. 700MB is much easier and quicker to download than 4GB. Alternatively, try the LiveCD - Mandriva One 2008 and you can do an install from here as well. This might be your better option.