If you have a copy of the latest version of Ubuntu (at the time of writing that is 8.10), there is a very simple program that does it all for you.
You will need:
* An Ubuntu 8.10 CD (or ISO image if already installed)
* A USB drive with at least 700-800 MB free space
While you are booted into Ubuntu, go to System > Administration > Create a USB startup disk.
Next, plug in your drive and choose it as your destination drive (and double-check it’s the right drive before going ahead).
You can also choose how much space will be reserved for saving data on the new drive, or turn data storage off (it will act just like the Ubuntu live CD).
That’s it! Simply press Make Startup Disk and wait while the files are copied over.
On many modern PCs that are configured to allow USB boot, you should be able to just restart the machine with the USB drive plugged in and Ubuntu should boot straight from it!
It’s a nice simple way to have a copy of Ubuntu in your pocket, wherever you go.