I think that, for a start, the feature you're asking for is not a feature, just a mere misunderstanding of the Windows installation procedure. I may not be an active Windows user, but I've gone through several installs when I, for some reason, needed Windows around. And both NT and non-NT offer the option to install on an existing drive.
In fact, one way to fix issues with windows is to install on an already existing file system, doing no changes. You do not lose any data in the
C: partition (not drive, what the OS sees are filesystems on partitions; some disks/drives may be formatted without partitions, that's for sure, but the usual is to have partitions).
You will only lose data in
C: if you tell windows to format it. So wait, you tell windows to format it, then you complain your data is gone. Perhaps you should not have told it to format the data, to start with.
Also, from what I remember, Windows NT separates user data and settings from the rest of the system, by offering a
Documents and Settings folder. So, if you really want to start from scratch you can just go there and delete the Windows folder, and keep Documents and Settings.
Once again, unless you tell the Windows setup to format the partition, you won't lose any data. One of the best ways to avoid shooting your own feet is not to fire the gun. And, in this case, you are trying to use a gun when a broom would suffice, or maybe there's not even a need for cleaning at all.
So this is not functionality you're asking for, you're asking for a way to use the same exaggerated workaround you did use in Windows. I'm afraid this can't be helped a lot. Just like Windows, even if GNU/Linux does partitions, if you tell it to erase everything and start from scratch, that's what it will do.
You can have separate partitions for, say
/home, that's the best way to achieve some kind of separation. But your comment of "If u can see.. the link shows 45.4gib allocated for /home, which will be earsed if I reinstall the OS.." shows that you are not understanding this at all. Ok, maybe the Ubuntu installer is so bad that it does not let you use existing partitions, but I thought it had a somewhat more advanced partitioning tool.
Once again, like Windows, there is an option to get rid of everything and start from scratch. You just have to read what is in front of you and not click in the option that will obviously get rid of your data...