There are several things going on here. You are correct in seeking clarification before proceeding, since your Windows partition is the same size as your target partition, and a mistake on your part will wipe out your windows installation. The sda1 is probably some sort of recovery partition, and not of interest, except that it takes up one of the four primary slots. Looks like the sda2 (10G=F) is a primary, and sda3 (157G=?) is the third primary, and it is probably the windows C, but lets not guess. Since you have more windows "disks"=partitions that cannot all be primaries, they must be "logical", so the last primary slot is made into an extended partition to hold many (256?) logical partitions. So sda4 is probably this extended partition, holding two logical partitions (E and C or D). The small difference in the number of G is a result of different measuring systems one based on 10 and the other based on 2 (2 to the 10th power is 1024, almost 10 to the power of 3). The simple fact is that the windows disk letters may be in any order, and even the sda partitions may not actually be in increasing sector order on the disk. The safest way to proceed is to examine your disks from the windows side (where you have D identified), Note where D appears, (I'd guess it's the 4th or 5th one listed), and free up its space right there in windows. Then the Ubuntu installer will see the free space, and try to use it. If not, at least you should know which partition from its position is D.