Various Linux distributions use different desktop environments.Ubuntu uses GNOME which is a lightweight environment that has moderately high system requirements.Other versions of Ubuntu called Kubuntu and Xubuntu use KDE(K Desktop Environment) and Xfce respectively.KDE is a bit on the high system requirements side but definitely not higher than Vista's requirements.It is around 180MB in size.Xfce is a lightweight desktop environment which amazingly has lots of features but is just 10MB in size.
In order to use both GNOME and KDE on your Ubuntu installation,all you need to do is to install the package kubuntu-desktop and all the packages that come with it.To do that,just run
sudo apt-get install kubuntu-desktop
You can also select to install the package 'kubuntu-desktop' from the Synaptic Package Manager located in the Administration sub-menu in the System Menu.
In case you wish to try out Xfce,you can also install the package 'xubuntu-desktop' by the same procedure mentioned above.
After doing that,you have multiple desktop environments.The next time you log in,click on change session and select GNOME,KDE or Xfce according to your choice and use the desktop environment you want to use.If you encounter any problems,refer to the Linux section of my blog for help.