If you open GParted you'll be able to see what partitions you have on your HD (GParted is available in the Ubuntu Software Center). You can see how your Ubuntu partition (or partitions, depending on how you did your installation) is formatted, how much free space - if any - you have, and if there are any extra partitions remaining from a Windows installation. Of course, if you have only one partition, it's Ubuntu (since you're running it).
Although Ubuntu can be installed on an NTFS partition, and you might have done that, if there are other partitions that are also on NTFS that might be a clue that they contain Windows data, but it's not a certainty. It is more common to format an Ubuntu partition with filesystems such as ext3 or ext4.
You can also open Nautilus (the second icon in the Unity launcher, which opens your Home folder) and see if other drives are shown on the left. Any drives you don't know about? If you wonder, open one and see what's in it. If you find folders and files inside, can you tell if some of them are from Windows?
If you do find a partition containing Windows, and you want to get rid of it, the easiest way is to use GParted to delete the partition that it's in. Of course, doing things with partitions is pretty much a one-way operation (yes, there are some tools to attempt recovery, but they're really for emergencies and not good to work into your plans), so you need to be sure before you commit.
And no, you don't need to defrag your Ubuntu partition, especially if you've used one of the Linux filesystems (such as ext3 or ext4).