I think it's worth pointing out that there are many of us who want to create a dual-boot machine so we can eventually migrate from the now defunct xp to newer windows or maybe Linux.
Not being familiar with Ubuntu/Linux we will want to have xp on one partition and ubuntu on another.
Then we can take time weaning from one to the the other, to see if it can cope with everything we need and if we like it.
We need an install choice that will see a blank partition and ask if that's the one for Linux (which obviously it nearly always will be).
I tried twice to achieve this including the choice for 'alongside xp' as suggested above. This time I installed xp on 39GB leaving the remainder of the 250GB disk for Ubuntu.
Unlike stated in the advice above, the install went ahead with no info given, or requested, and placed itself on the 39GB partition, with XP, even though it had seen the empty, this time unformatted, partition.
The Ubuntu guys really need to get their act together and modify the 'installation Type' page so that people unfamiliar with this stuff are better enabled to make the required choice easily.
Currently there is no way non-Linux users can predict the outcome of each choice.
If this issue is addressed, I may try again but there is currently nothing easily available that helps.
/dev/sda
(the default option).