I have Windows Vista and Ubuntu 10.10 on my Laptop !

How can I use Microsoft Telnet to connect to Ubuntu ??

Is telnet somewhat like the Linux Command Prompt running on windows ??
Please Help !

Telnet is a clear text protocol and therefore is usually a *bad idea* .. most linux distros these days will not have a telnet server enabled out of the box. However most will have sshd enabled so your best bet is to get a ssh client like PuTTY or similar and use that to connect.

If both OSes are installed on disk in the same machine and you are dual booting, then only one can run at a time, so you can't do this.
If however, you install one as a virtual machine on the other, you can. For Ubuntu specifically, also google 'wubi install'.

Telnet is indeed an insecure protocol, but on your own PC isn't a big deal.

PuTTY is good, installing Cygwin (http://www.cygwin.com/install.html) gives you regular SSH and a mostly functional BASH shell. It is the first thing I install when I am forced to use Windows, as it gives you POSIX platform and build chain (i.e. gcc, make, etc.), as well as a package manager with most of the "usual and customary" console tools and services most of us think of as routine (but which are still missing from Windows 7, go figure).

You cannot, however, do what you want without some additional efforts. You need to have Linux (Ubuntu or otherwise) running someplace, not just installed. The easiest way (IMHO) to do this is to set it up in a virtual machine (VM). Microsoft makes something, but I use VirtualBox (http://www.virtualbox.org/wiki/Downloads) since it is free (as in $0 for personal use, FLOSS version still available last time I checked, but now that Oracle bought Sun who bought VirtualBox, only time will tell if that lasts) very easy to set up and use. If you have a console-only VM you can ssh into it. You can even do things easier, since you can have the whole Linux VM run in a window, like any other application.

You can also do the reverse when you are booted into Ubuntu--install a Windows VM (I have an old WinXP VM I use on my home PC for a few applications that still won't run under WINE).

Personally, I use KDE 4.7/openSUSE 11.4 and run my VM in full-screen on my work-issued Win7 laptop so I don't even have look at Windows most of the time. I don't have any sort of experiences that would suggest that GNOME wouldn't be the same.

One other nice thing is that you can (with a little extra disk-space, of course) create multiple VMs one with Kubuntu, one with GNOME, one with Unity, one with Xfce, one with Sugar, one with...(you get the idea) and compare them side-by-side. You can also partition and format a thumb-drive with Ext and carry your VM around with you. Just be sure to encrypt /home in case you loose it. You cannot use FAT32, since you will likely have a virtual drive >2GB.

You can easily learn and use telnet to act like a remote command line access.

If you want to run remote x then might look at mobaterm.