Enter that at a command prompt. Actually it should probably be

[I]sudo[/I] apt-get install xubuntu-desktop

Or if you're more of a GUI kind of guy, you can search for the package through Synaptic Package Manager and install it from there.

Use the Terminal application. On Gnome I believe this can be accessed from the Gnome menu->Utilities.

or do control + alt +f2

Is there a way to go back to Ubuntu after the install?

After installing xubuntu-desktop, you should be able to choose between an Xfce and Gnome session in your login window. The former is for Xubuntu, the latter, Ubuntu.

The other thing is...

When I press Control + Alt + F2... It takes me to a black screen where it asks me for myusernameand password. After I give my Username and Password... It says a whole bunch of stuff like "Ubuntu is not under Warranty.." Then nothing else happens. What do I do?

That means that you're at the command prompt. It's waiting for you to type a command for it to run.

Two things...
One: I can't see if I'm typing anything...
Two: When I press enter nothing happens after I type...

What should I do?

Is there a flashing cursor? Do you see anything that looks similar to this?

[username@host ~]$

Nope... All I see is My Last Login...
- The programs included with the Ubuntu system are free software; the exact distribution terms for each program are described....

The the last line is..... " Ubuntu comes with ABSOLUTELY NO WARRANTY"

The the screen stops..

No flashing cursor or anything...

The only time I can get it to do anything is when I press Control + Alt+ Delete.. and the computer turns off...

Any idea???

Just so you know, you can get back to the GUI by pressing Ctrl-Alt-F7.

To be honest, I'm not really sure why you aren't getting a prompt... what happens if you start up the Terminal application from within Gnome?

How do I do that? I looked all over for a nome Menu and I cannot find it...

My bad. The Applications menu->Accessories->Terminal.

It worked I got to the prompt and I typed the

sudo apt-get install xubuntu-desktop

And it said..

E: Couldn't Find package xubuntu-desktop

???

What should I do?

Run

apt-get update

before you install xubuntu-desktop .

Do i need an internet connection to do this

Well, I'll update you once I get a network card...

Ok... Got a card...

So, I followed your instructions...

I went

apt-get update

and it said access denied

So i typed

sudo apt-get update

and it did something like

Ign cdrom:// Ubuntu 7.10_Gusty Gibbon..........Main translation-en US

So i continued with

sudo apt-get install xubuntu-desktop

and it gave me the same error as before

E: Couldn't find package xubuntu-desktop

Any ideas..Thanks

What is the contents of /etc/apt/sources.list ?

I don't know what to tell you....

I opened it up and it says "Software Sources"

and its giving me some options...

No, start up a text editor like gedit, open /etc/apt/sources.list and post the contents of the file here.

is gedit in Ubuntu or do I have to download it?

Well, gedit is probably installed somewhere on your system. If you can't find it, use vim. It's probably easier to use anyway. Just type

sudo vim /etc/apt/sources.list

And vim will start up. You're all set! Here's the vim man page if you need any help:
http://www.eandem.co.uk/mrw/vim/usr_doc/index.html

gedit and nano are both present in standard ubuntu installs