First, Microsoft developed Microsoft Security Essentials because their
software/operating systems are full of bugs and easily exploitable!!
So they need Microsoft Security Essentials just to keep your computer
(and their software) relatively safe on a day to day basis.
Linux software is very different in many ways so the possibilities of
viruses, trojans and other exploits are very small.
I would suggest downloading Ubuntu and trying it on your laptop
for a couple of weeks to see if you like it. I think you will.
If you need/prefer a desktop that looks like windows try installing cinnamon
desktop on your Ubuntu or perhaps try another Linux distro that has
Cinnamon or Mate as one of its default installs. (Try Linux Mint or PCLinuxOS)
As for Microsoft Word, I think that you will find Libreoffice to be
an excellent and nearly identical substitute for it. You can even open
and edit Word documents with Libreoffice.
Thunderbird is a great email program that you might like a lot. It is
made by the same people that make Firefox.
A good place to see a list of the other Linux distros is distrowatch.com
But I really like Ubuntu because I found it to be extremely stable and
its support is fantastic.
Personally I am using Ubuntu 12.04 with the Cinnamon desktop on 2 computers
and 3 laptops because I just really like it.
Here is a link to a helpful thread about the different desktop environments
you can use
What different desktop environments and shells are available?