OK, the  (almost) painless way to install Ubuntu from a USB optical drive starting from the Windows OS. If you feel any of this would be useful to your visitors feel free to add it to your page.  I'll likely post this on one of my sites eventually but I'm in no hurry to do so.  If I do I'll send you the link. 

Here's how I did it on my P1120 using an Iomega ZipCD (USB) 650:

   1. Download and burn the latest ISO (currently it's ubuntu-5.10-install-i386.iso).  I ended up having to install Bittorrent just to get the image because FTP kept timing out -  even in the wee hours of a Sunday on DSL. If you do use Bittorrent just make sure you download the right Ubuntu version for your instlux installer below.  
   2. Download the corresponding instlux installer from Sourceforge - http://sourceforge.net/project/showfiles.php?group_id=151507&package_id=167524&release_id=373706 . For Ubuntu 5.10 the installer is instluxUbuntu5_10.exe
   3. Plug in the optical drive, put the Ubuntu CD in it, and from Windows, run the CDROM install option on instlux
   4. System will reboot, choose the option that says "linux" from the bootloader
   5. Ubuntu install should detect your USB drive and run Ubuntu installation from your CD without incident. There's a good Ubuntu install walk-through here: http://users.bigpond.net.au/hermanzone/p3.htm that includes detailed information on using the included partitioner to resize your Windows partition.
   6. Next time you boot to Windows, you'll get 2 bootloaders - one from the Ubuntu install and another from the instlux installer. Once Windows starts, instlux will automatically prompt you to uninstall the instlux boot loader.

Some partitioning notes for dual-boot XP/Ubuntu:

Not many (safe) options exist for resizing NTFS partitions unless you lay out some cash for Partition Magic. The Ubuntu install walk-through mentioned in #5 above shows you how to use the Ubuntu partitioner to resize your Windows partition and make room for Ubuntu. Unfortunately I can't verify this as I didn't see a resizing option when I installed Ubuntu on either of my systems; but I'd already created free space on each drive, and it's possible that option only exists if Windows takes up the entire drive. I used the (GNU) GParted LiveCD - http://gparted.sourceforge.net/ - on my Portege to resize the  Windows partition and create the free space prior to the Ubuntunu install -  download the GParted LiveCD iso, burn it, put in the CD, change the BIOS to boot from CD, and reboot.  After resizing Windows to create free space, I ran the Ubuntu install CD and partitioned the free space using the recommendations from the link in #5 above. While I didn't have to run Gparted to create free space on the P1120 - I'd already created a second partition when I replaced the old drive* -  it should be possible to run the GParted LiveCD using the instlux boot loader to detect the USB drive prior to running the Ubuntu install CD.

*The P1120 comes from the factory with a DISE partition that can also be fiddled with to install Ubuntu if you don't mind deleting the .PQI files from the partition. Back them up first incase you ever need to restore it to factory settings. You'll need DriveImage 2002 or later to restore the .PQI image as it seems any changes you make to the drive will hose the original DISE installation anyway; even backing up the drive and re-imaging with a different utility or to a new drive seems to hose it.