The Guided option would allow you to chose the size of the partition.

I have read somewhere that it is better to install XP last. (AFTER Umbuntu) I
did that and had no problems.

And that worked? That goes against everything that I have been taught.
But, if it does work, great!

Well I think it just might if you have two hard drives or if the XP is
you are able to get the XP to even see the Liux partition but igf you
did it good luck to you, Like Wade it flys in the face of our practice
but the this is the world of computers.

I did just the opposite, and in fact I've installed Ubuntu
and Kubuntu several times, removed partitions, etc., and I'm THANKFUL
to say it all works, including my original XP partition.

The rule of thumb here. MS Windows does not play nice with any
other Operating System on the hard drive unless it is MS windows. Thus you need
to install windows first and Linux second. If you Google search
'Dual Booting Ubuntu 7.04" you'll get much needed information on how to do this.
Sorry to tell you that MS does not play nice. Why use some thing for free when
MS rules the world.

If Windows isnt at the front or top or first position on the hd, I think
it starts to write data incorrectly. Its been a while since I was
working on this but, I think I would be really really careful with my
data on windows at this point. It might be working but I would imagine
its not stable.

That's why I like windows in second place...It doesn't have a chance to get its
hooks into the system when I use linux.

But if you edit the boot sequence you can get either to be first in the
list so it matters not how you load them but XP is dominant and cannot
natively read linux so for safety sake its always XP first.

I have both Ubuntu and Linux Mint installed on my laptop. The way it's
set up, Linux Mint is the default OS (I'm not sure I'm using the right
terminology here, so bear with me). I want it set up so that Ubuntu is
the OS that boots by default. How do I do that?

This is how to edit grub and change the boot order.
https://help.ubuntu.com/community/GrubHowto/ChangeDefaultOS

Note: in this example they are changing it so Windows XP is the default, but the
principle is the same.

In your situation, you want to change the default number to whatever the
corresponding number for Ubuntu is. If Mint was the default it would have been 0
and Ubuntu 1. Just change the Default 0 to default 1 and save it.

An alternative is to cut and paste the Ubuntu section and place it ahead of Mint
section and leave the default at 0. Either should work. Don't forget to save
your changes.