Linux

Download this one

download.virtualbox.org/virtualbox/4.2.10/VirtualBox-4.2.10-84105-Win.exe

And I asked for whether its a 32 bit or 64 bit processor. which u can see by rt clicking --> Properties on my computer in Windows.

Mine is 32 bit.
So I am downloading the one you have mentioned.
I would update you regarding my progress.

---------- Post updated at 07:31 AM ---------- Previous update was at 07:14 AM ----------

Just clarifying one point: I haven't gone through the other document named 'how to install virtual box ...' but would like to clarify that the existing 2 OS that are installed (UNIX AND WINDOWS) in my system needn't be removed? i.e., I don't have to again install the 2 OS?

Yes it's downloaded successfully.
Next, when I simply opened it and it asked for installation. I clicked yes and it started installing. (Yesterday I downloaded one link for the fedora 16 as mentioned in the oracle website but when I opened it , it didn't install. and a message came that : windows can't open, search from net or use other appl. to open. From this it's a good signal that the file you provided to download is the right one.)
As per the document "how to install oracle VM Virual .....", I need to 1st add 'mirror' in /etc/apt/sources.lst. That means, in Linux I need to add? and if so which one to be added among the options available?
Adding that without following the document, when I started installing the downloaded file by pressing 'next', I got the output as displyed in the screenshot 'oracle1'. From this screenshot its clear that it isn't installed properly.
Also, while installation when it asked me to install one application as shown in the other screenshot named 'oracle', I denied.

Also, the link to download the public key took me to google search site where many links where there. So, do I need to check in those sites one by one to get the key?
After your reply, I w'll get direction to proceed.
Thanks and waiting for your reply

No need to uninstall any of the OS. Using VM you can run buth the OS at a time.

And ur later post issues regarding Installation of Virtual box...

I did it for Open suse and it worked fine for me. Need to see the issue U faced while installing . I will look into the issue and get back to you asap.

Please find the document you had sent and clarify me what needs to be done and where as it's mentioned "First, add one of the following mirrors....."
Then please clarify the public key point also.

Did it ask for any mirror or something while installing? and also any key?
U can go for a default installation allocating HD and ram size. If it asks for any DL mirror please paste the screenshot which will be helpful to analyze.

Actually i did it for OpenSUSE and it didn,t give any hurdle while installing.

Yes it installed successfully. The issue with my earlier installation was that while installing the virtual box, when it asked me to install an application, I denied (I had mentioned you regarding this in my post 5 days back where 2 screenshots are attached). This time I accepted and it successfully installed.
Thanks a lot for your support.
Now, I would proceed ahead to create virtual machines and update you regarding the progress.