Im thinking of getting linux - i have a few questions

Which version should i get? I think Ubuntu, but im not sure.

Whats the difference between windows and ubuntu?

Will i still be able to use all the programs i already have installed and access all my documents?

Linux is a completely different OS to windows . You won't natively be able to access your programs, but as for your documents you should be able to access them just fine.

Ubuntu comes packed full of most of the programs you'll will find yourself using, and new programs are very easy to install. (most are found in Debian repositories). Linux itself is more secure, however the extra services ubuntu add can make it slightly less secure. (don't worry about this however )

Theres 3 flavors of ubuntu;
zubuntu - being a lighter version for lighter computers

ubuntu - default
-http://polishlinux.org/stuff/screenshots/ubuntu/ubuntu_dapper_desktop_nautilus_libervisco.png

kubuntu - slightly heavier, looks a lot like windows.
-http://www.zimmertech.com/images/kubuntu_full.jpg

try it out first, see if you like it. If not you can always uninstall

Ok, will programs like:

itunes, photoshop, firefox and windows messenger still work, or will i have to re-install them?

itunes can be scraped for rhythmbox, i haven't seen any difference and it lets you drag and drop songs onto your ipod.

photoshop, your stuck using GIMP

firefox - available in linux

windows messanger - ubuntu uses pidgin instead.

Also note WINE can emulate windows program under linux.

will wine let me use all my programs i have now as normal?

It can use quite a few, but I would google "<program name here.> WINE" just to be sure.

If you're using Windows at the time, no. There are tons of programs that do not work on a Linux system.

I would just like to note that WINE takes some getting used to and only emulates windows applications. I personally found it hard to use and didn't find it that useful for my needs.