Because you don't have 2 hard drives. You have a 36.5GB drive and a 4GB drive(a flash drive?

) The 36.5GB drive has 2 partitions, a 15GB and a 20GB. The 15GB is formatted NTFS, and has Windows XP. The 20GB is formatted FAT32, and is empty. Delete that partition using the PCLOS Control Center on the LiveCD. Then, upon entering the installer, choose "Use remaining free space on Drive" or something like that. The installer will automatically create your partitions. But those partitions would probably not be very useful, considering your limited drive space, so you should preferably go into Custom Partition and create your partitions. If you have less than 2 GB of RAM, then create a "swap" of double your RAM. If you have 2GB or more of RAM, then create a "swap" a bit larger than your RAM. After that, use the remaining space for a "/" partition.