[Rahim]

Dont panic alsiladka. Its very easy, Believe Me.
When on the Partition page in Ubuntu install, choose "Manual" and and the Partition will be shown like this:
Vista partition = sda1
2nd Partition = sda5
3rd Partition = sda6

You are concerned with 2nd one as the 3rd is your Backup partition.
If you are confused with your partitions' names then write down their sizes.
Here we go:
1: Choose sda5(ur 2nd partition) and create a "/" (Root) Partition, where your Ubuntu will be installed, preferably 39 GB, ext3 type and Choose mount point "/".
2: Use the remaining 1 GB as Linux-Swap.
Thats it.
Just to make sure that Grub is installed properly ,on the last page(Summary Page) of the installation, on the bottom-right side click on "Advanced" and make sure (hd0,0) is written there. It will write Grub into the 1 Hard Drive(hd0) into the 1st Partition(0).
Now when it is installed , it will reboot and you will be presented with The Grub Menu with Ubuntu & Vista entries. Choose your OS and have fun.

PS: What about the link i gave you in my previous post. Are you not following the steps? Clarify plz.