There are several steps to follow to accomplish a dual boot system. I would bet the entire DH is allocated to windoze. You will fist have to find out exactly what partitions are there.
O.K. a partition is a division in the disk, allocating space. In windows you would know this as C: D: etc. To complicate matters the terminology in linux is different. We designate partitions by drive, and partition number. For example, the first serial HD would be sda. The first partition would be known as sda1. The second as sda2, etc. Then next drive would be sdb, so the fist partition on the second drive is sdb1 etc...
A little more about partitions. There are different kinds of partitions. There are primary partitions, and extended partitions. You are only allowed 4 primary partitions on a HD, no matter how big it is. This is why extended partitions exist. If you need more than 4, we usually create a primary, ( one is necessary ) and the rest in extended. An extended partition can be sub-divide into more partitions. Not sure what the limit is, but many. You can also "hide" partitions. This is often used on netbooks. Since Bills company no longer supplies CD's to install from, there will be a restore partition you can not see. There is probably a diagnostic partition as well, hidden again. This means the manufacturer has used up all the primary partitions on you. I know, I have a HP netbook that came this way.
In short, what I'm telling you is, you need to find out what you have available, not unused space as windoze sees it. Unused space that is allocated to windoze or a restore partition, is not available to install linux to.
I had no interest in windbloze, so I just deleted the partitions, but new ones and installed. Bye windoze! I know you do not want to do that.
What to do. There are distros that supplies visual tools to allow you to 'see' the partitioning scheme. One is called Partedmagic. It is bootable from a CD. Netbook, no CD drive I bet. You may be able to install it to a USB stick, and boot. Then you will be able to see the scheme you have bought.
There are also windoze programs like Partition Magic that do the same things. Your choice here.
You are part way there. The first thing to do is backup any windoze stuff you do not want to loose. Second in defragging. Next is to start shrinking at lease one windoze partition, to create free un-allocated space. Next move that space ( your 50 or so gig ) beyond the windoze partitions, and any hidden partitions. Then you can install to that unallocated space. Ubuntu has partitioning tools in the install, so you can create your linux partitions. BTW, the reason for moving the free space beyond windoze, is , windoze may not be able to recognize any partition that is not a Win part, and would no longer 'see' a partition beyond a linux part.
There is another way. Personally I don't like it. Ubuntu has something called Wubi. As I understand it, you install Linux within your windbloze file system, using Wubi. Never used it myself, since I don't trust windoze NTFS file systems.
This was created for noobs so you can install, run linux, learn it, and then do what you like later. ( a real install ).
You can find out more about Wubi on the Ubuntu forums. See this link -->http://ubuntuforums.org/
Ask questions, it took me a long time to understand some of this stuff, there is a lot to know...
Hope this helps.