xp laptop, can't boot from usb. use raid w/ hdd?

Yes. Ubuntu installer (Ubiquity) will install Ubuntu boot loader (Grub2) on first sectors of XP disk, and this last one will load Linux kernel from external drive. But you will need external drive to boot XP, as some part of boot loader will be installed in "/boot" folder of Ubuntu.
To avoid this, you could make a dedicated "/boot" partition on XP disk. If you choose this solution, post again, and I will provide more data.

Oliver,
thank you. I do need to make a dedicated "/boot" partition on the xp disk. So, if you please, I'd appreciate you providing me with the instructions.
Genuine thanks.

Boot on Ubuntu installation media.
Choose "Try without install" instead of "Install". You will get a live session.
Open a terminal (Ctrl+Alt+T)
Type "sudo gparted".
Gparted will display partitions of hard disk.
Select NTFS partition which hold Windows.
Do a resize on it, to make free 1GB at right.
On this freed space, make a new ext4 partition, and format it.
Don't forget to click on green arrow to run commands (gparted uses a "arm and fire" system).
Then install Ubuntu. Choose manual partitioning.
On internal disk, mount "/boot" folder on new little partition.
On external drive, make a 2GB swap partition.
After you have choice to make one ext4 partition, on which you will mount folder "/".
Or make
a) a 10GB ext4 partition on which you will mount folder "/"
b) with remaining space, make an ext4 partition on which you will mount folder "/home"
The second solution, will allow easer re-installation or installation of Ubuntu release (or another Linux distro).