Are you sure you cannot boot from CD? When your computer starts, it should say something like 'Press F12 to enter System Configuration', which will take you to the BIOS settings. There, change the order of bootable drives so that the CD-ROM drive is booted before the Hard Drive. Then, save settings and exit, put in the CD and see if it works.
If you need help installing Mandriva (although I don't think you will), check out this tutorial
Best of luck,
Ps. I'm pretty sure there are no floppy installation capabilities with Mandrake/Mandriva (there were for Mandrake 7.1, but that's quite old). There are boot floppies for Debian though, if you're interested. I'd advise sticking with the CDs if you can.