Im a bit confused here please correct me if Im mistaken.

A. When installing ubuntu from windows it creastes a double boot but the installation sits on a Windows partition. right?

B. When installing ubuntu ( booting up from the CD ) and u let ubuntu Resize the partition it creates a double boot aswell but it does install ubuntu on a linux partition. right?

and another question... hehe does ubuntu works slower reading and writing on NTFS partitions ?? or is it about the same as using a Linux Partition format?

If you are talking about the WUBI installer, then Ubuntu gets installed to a file on the Windows partition. I've not done it, but I read it adds an option to the Windows boot-loader to boot to Ubuntu.

According to the official site FAQs (http://wubi.sourceforge.net/faq.php)...

"""
What is the performance?

The performance is identical to a standard installation, except for hard-disk access which is slightly slower than an installation to a dedicated partition. If your hard disk is very fragmented the performance will degenerate.
"""

If you boot from a CD, then you would need to create partitions for Ubuntu to install to.

You can do this, but it requires some extra program, the name of which currently escapes me (maybe included in the latest Hardy version???). Left to its own devices, it will only install into some sort of linux partition.

(I'm assuming by double boot you mean dual boot? Can't see what else you could mean.)

Yes, but it only works if it can create a big enough partition and it can only do this if there is enough free space at the end of the existing partition. So you may need to / want to defrag first.

I've not tried, but its probably slightly slower. But given that the question "Which is the fastest linux partition type?" can cause arguments going on for ever and ever, with everyone saying that under this that or the other load profile, they have a test to prove that their favourite type is best, don't expect a definitive answer.

And I don't think NTFS supports all the file attributes that a Linux filesystem really wants, so it doesn't work as well as a proper filesystem would. And NTFS write spent a lot of time marked as "experimental" so it probably isn't as reliable (although whether that would bother you if you didn't have a server...).

If it is to share files (such as pics, music, movies) amoung operating systems why not make a fat 32 partition for storage. Both systems will read and write.