Can I run Ubuntu off a usb - pen? How?

Here's a guide that should help you get it sorted...

http://www.pendrivelinux.com/2008/05/15/usb-ubuntu-804-persistent-install-from-linux/

this is one way........if your using a linux operating system:

Prerequisites for installing Ubuntu 8.04 to a flash drive:

    * Working Linux environment (we used a local Ubuntu installation)
    * Established internet connection
    * 1GB or larger USB flash drive

How to install Ubuntu 8.04 to a USB flash drive from Linux:

   1. Insert a 1GB or larger USB flash drive (2GB+ recommended)
   2. Open a terminal and type sudo su
   3. As one line, type wget releases.ubuntu.com/releases/8.04/ubuntu-8.04-desktop-i386.iso
   4. Type mount -o loop -tiso9660 ubuntu*.iso /cdrom
   5. Type fdisk -l to list available drives/partitions (note which device is your flash drive I.E. /dev/sdb). Throughout this tutorial, replace all instances of x with your flash drive letter. For example, if your flash drive is sdb, replace x with b.
   6. Type umount /dev/sdx1
   7. Type fdisk /dev/sdx
          * type p to show the existing partition and d to delete it
          * type p again to show any remaining partitions (if partitions exist, repeat the previous step)
          * type n to make a new partition
          * type p for primary partition
                o type 1 to make this the first partition
                o hit enter to use the default 1st cylinder
                o type +750M to set the partition size
                o type a to make this partition active
                o type 1 to select partition 1
                o type t to change the partition filesystem
                o type 6 to select the fat16 file system
          * type n to make another new partition
          * type p for primary partition
                o type 2 to make this the second partition
                o hit enter to use the default cylinder
                o hit enter again to use the default last cylinder
                o type w to write the new partition table
   8. Type umount /dev/sdx1 to unmount the partition
   9. Type mkfs.vfat -F 16 -n ubuntu8 /dev/sdx1 to format the first partition
  10. Type umount /dev/sdx2 to ensure the partition is unmounted
  11. Type mkfs.ext2 -b 4096 -L casper-rw /dev/sdx2 to format the second partition
  12. Remove and re-insert your flash drive (if prompted that a new medium has been detected, select to open in a new window and click ok)
  13. Back at the terminal, type sudo apt-get install syslinux mtools
  14. Type syslinux -sf /dev/sdx1
  15. Type cd /cdrom
  16. Type cp -rfv casper dists install pics pool preseed .disk isolinux/* md5sum.txt README.diskdefines install/mt86plus /media/ubuntu8
  17. Type cd /media/ubuntu8
  18. Type wget pendrivelinux.com/downloads/u8/syslinux.cfg
  19. Type cd casper
  20. Type rm initrd.gz
  21. Type wget pendrivelinux.com/downloads/u8/initrd.gz
  22. Reboot your computer and set your system BIOS boot priority to boot from the USB stick.

You should be able to boot Ubuntu 8.04 from the memory stick and by default it should save your changes, restoring them on subsequent boots

this is one way if your using a windows operating system:

you should get this usb installer first. go to to this site -->https://fedorahosted.org/liveusb-creator/

then download the Ubunto iso image and use the usb installer it install the ubuntu iso image to the usb flash stick.

hope this helps.....someone else may have something better..........i installed fedora from windows vista using this method.