Ubuntu on a flashdrive

(from: http://www.howtogeek.com/howto/14912/create-a-persistent-bootable-ubuntu-usb-flash-drive/)

 Create a Persistent Bootable Ubuntu USB Flash Drive

Don’t feel like reinstalling an antivirus program every time you boot up your Ubuntu flash drive? We’ll show you how to create a bootable Ubuntu flash drive that will remember your settings, installed programs, and more!
Previously, we showed you how to create a bootable Ubuntu flash drive that would reset to its initial state every time you booted it up. This is great if you’re worried about messing something up, and want to start fresh every time you start tinkering with Ubuntu.
However, if you’re using the Ubuntu flash drive to diagnose and solve problems with your PC, you might find that a lot of problems require guess-and-test cycles. It would be great if the settings you change in Ubuntu and the programs you install stay installed the next time you boot it up.
Fortunately, Universal USB Installer, a great little program from Pen Drive Linux, can do just that!
Note: You will need a USB drive at least 2 GB large. Make sure you back up any files on the flash drive because this process will format the drive, removing any files currently on it. Once Ubuntu has been installed on the flash drive, you can move those files back if there is enough space.
Put Ubuntu on your flash drive
Universal-USB-Installer.exe does not need to be installed, so just double click on it to run it wherever you downloaded it. Click Yes if you get a UAC prompt, and you will be greeted with this window. Click I Agree.

In the drop-down box on the next screen, select Ubuntu 9.10 Desktop i386. Don’t worry if you normally use 64-bit operating systems – the 32-bit version of Ubuntu 9.10 will still work fine. Some useful tools do not have 64-bit versions, so unless you’re planning on switching to Ubuntu permanently, the 32-bit version will work best.

If you don’t have a copy of the Ubuntu 9.10 CD downloaded, then click on the checkbox to Download the ISO. You’ll be prompted to launch a web browser; click Yes.

The download should start immediately. When it’s finished, return the the Universal USB Installer and click on Browse to navigate to the ISO file you just downloaded. Click OK and the text field will be populated with the path to the ISO file.

Select the drive letter that corresponds to the flash drive that you would like to use from the dropdown box. If you’ve backed up the files on this drive, we recommend checking the box to format the drive.

Finally, you have to choose how much space you would like to set aside for the settings and programs that will be stored on the flash drive. Considering that Ubuntu itself only takes up around 700 MB, 1 GB should be plenty, but we’re choosing 2 GB in this example because we have lots of space on this USB drive.

Click on the Create button and then make yourself a sandwich – it will take some time to install no matter how fast your PC is.

Eventually it will finish. Click Close.

Now you have a flash drive that will boot into a fully capable Ubuntu installation, and any changes you make will persist the next time you boot it up!
Boot into Ubuntu
If you’re not sure how to set your computer to boot using the USB drive, then check out the How to Boot Into Ubuntu section of our previous article on creating bootable USB drives, or refer to your motherboard’s manual.
Once your computer is set to boot using the USB drive, you’ll be greeted with splash screen with some options.

Press Enter to boot into Ubuntu. The first time you do this, it may take some time to boot up. Fortunately, we’ve found that the process speeds up on subsequent boots.

You’ll be greeted with the Ubuntu desktop.

Now, if you change settings like the desktop resolution, or install a program, those changes will be permanently stored on the USB drive! We installed avast! Antivirus, and on the next boot, found that it was still in the Accessories menu where we left it.

Conclusion
We think that a bootable Ubuntu USB flash drive is a great tool to have around in case your PC has problems booting otherwise. By having the changes you make persist, you can customize your Ubuntu installation to be the ultimate computer repair toolkit!