dual boot - How to Install Ubuntu by replacing Windows but without formatting other partitions

I'm using Windows 7 on my PC (1 GB RAM,160 GB HDD).
I want to install Ubuntu and remove Windows 7
I have Partitioned my PC into 2 drives , C Drive - 40 GB and D Drive - 120 GB.
I want to install Ubuntu on the C drive without formatting D drive so how can I do that?
I know that I will have to select the Something Else option when the Installation Type Screen appears but I don't know how to proceed from there.
My question looks kinda similar to this question but its not the same.

Actually it's very simple to do that

I'll start from the something else option because you are asking about that.

• Click something else and then choose Continue




• A window will open something like:

  
  

  

  
  

  in your case it would be different and will display two partitions sda1 and sda2 under sda. And also the free space would be at the beginning ;)




• Select sda1 and click Delete it will free the space at the beginning.(The picture is just for help, it is at the last in the picture).




• Then again select the free space that you've made free and click Add. A window will open something like:

  
  

  

  
  

  choose the exact value except the size. You have to set the size according to your need and you've to leave some size for Swap space. So I suggest you to leave 4GB for Swap. So allocate around 36 GB of size, click Ok




• In the next step again some free space will be left, so select that and again click Add button. this time you have to allocate all the free left space to Swap. You have to chose Use as: option in order to assign that as Swap.




• That's all you have to do. While highlighting the sda1 to which you are going to install Ubuntu just click Install Now button.