How did you install 98SE? Most likely its installer overwrote the master boot record, so your computer boots only to it now. You can (possibly) fix it by typing fdisk /mbr in MS-DOS mode. This will restore the MBR to its original state, which means it won't go to 98SE. You can then add it to Mandrake's bootloader.
If, however, the installation screwed up your partition table, it's more complicated to restore it. Try the above suggestion first, post back if it doesn't work.