Unless you have one primary partition for your Linux partition, there isn't much you can do. So you need to delete one of your existing partitions before you can install Ubuntu.

Unfortunately the disks are limited to 4 physical partitions, but if you need more, then you can set up the 4th partition as an extended partition. Inside that extended partition you can have several other logical partitions.

But as mentioned earlier, you need to delete one of your existing partitions before you do so.

Here is a sample layout: Partition table

Hope that helps!