I have read many tutorials regarding dual boot, but it's mostly about having 1 partition of OS (ex. windows) and the users have to create a new partition.

I'm really not familiar with those stuffs so I don't understand the tutorial that much to apply it to my case. Because I already have 2 partitions, The first one has 50G where my Windows OS is, and the other has 100G where my other stuff is (It has 50G free space left).

I'm really unsure how to apply what I see in the Tutorials (from youtube and google).

I'm pretty sure though, that there's an easy way to install my backtrack (I already burned it to a cd), because I already have a second partition it can use. I just don't know exactly how.