How can I create a Live USB with all the software that is installed on my computer?
Reconstructor works really well to make a customized Ubuntu Live Distro. I am not aware of a easy way to turn your current install into a Live USB...
Reconstructor is really easy to install with a internet connection:
sudo apt-get install reconstructor
After it is done installing it should be in the Applications menu under System Tools.
Another good tool is Remastersys, which looks like it may do what you need, but I have never tried it...
I am going to give it a try and see how it works.