There are so many tutorials online on how to crack WEP i.e. Wired Equivalent Privacy and I have tried quite a few of them before I finally settled for one that actually worked for me. I have re-written the tutorial from remote-exploit.org with some screen shots.
Just a couple of things to note before you delve into this tutorial

1. This tutorial is for information and educational purposes only
2. Use your own wireless network to perform the steps in this guide
3. I hold no responsibility for how you use the information in this page

NB. The screenshots thumbnails are linked to larger more legible images.

First Steps - Downloading Backtrack and aircrack

• Download BackTrack live CD from the closest mirror at http://www.remote-exploit.org/backtrack_download.html

• Download aircrack from http://download.aircrack-ng.org/aircrack-ng-0.9.3.tar.gz and keep it handy e.g. save it in a USB stick.(you’ll need this later)

• Burn image to CD using Nero or any capable application.

• Reboot your system from the CD and wait as the system loads

NB. YOU’LL NEED TO DO A BIT OF TYPING TO EXECUTE PRETTY MUCH ALL OF THE COMMAND, BUT YOU ONLY NEED TO TYPE THE TEXT I’VE TYPED IN SQUARE BRACKETS AND REMEMBER TO TYPE ENTER AT THE END OF EVERY COMMAND

Launching Backtrack and loading X-server

4. Once the system has loaded, Login as “root” using the password “toor” without quotes by typing the following

bt-login: [root] <Enter>
Password: [toor] <Enter>

Once logged in, you’llbe taken to a command prompt, you’ll need to launch a graphical user interface(GUI), you do this by issuing the following command

bt ~ #[startx] <Enter> (see p1.jpg)

p1.jpg

then wait for the GUI to load(see p2.jpg)

p2.jpg