So you didn't delete Mandriva and kept the partitions untouched? Hmm, if that was a good idea
Can't you just re-write the bootloader using YaST? Check if the settings are correct (point to the correct partition) and write it to the MBR again. Otherwise you may want to try grub instead of lilo. This may solve troubles with leftover configuration files from Mandriva.
If you have re-arranged partitions, again make sure that what used to be your root partition still is called hda3. Good luck!