How do I install ubuntu on a secondary partion.

Ubuntu will install itself into the second partition, however, it will split up a bit of it for swap space. Ubuntu will override the master boot sector and setup grub as the multi boot menu. It is possible there to change the default OS to be loaded to be windows. All of this is done more or less automatcially during the installation from the live-CD.

Here is some help concerning this:

https://help.ubuntu.com/6.10/ubuntu/installation-guide/i386/index.html

Here a screencast to watch the steps:

http://doc.ubuntu.com/screencasts/Installing_Ubuntu_with_Windows_Dual-Boot

I hope this helps

Hello. I´m new in this issues. I have a DELL Latitude 620 with 80Gb HDD Laptop. I want to have a dual boot laptop (Windows XP and Ubuntu), but the HDD has just one partition. What do you recomend to install Ubuntu 7.04? What I have to choose when the installer ask about the partitions of the HDD?
Is OK to choose the automatic installation? Will I be able to use my lapton with Windows XP, after this?
Thanks in advance. Regards.

Sorry, I used the wrong button.

Hello. I´m new in this issues. I have a DELL Latitude 620 with 80Gb HDD Laptop. I want to have a dual boot laptop (Windows XP and Ubuntu), but the HDD has just one partition. What do you recomend to install Ubuntu 7.04? What I have to choose when the installer ask about the partitions of the HDD?
Is OK to choose the automatic installation? Will I be able to use my lapton with Windows XP, after this?
Thanks in advance. Regards.

@Damián

If not all of the HDD is used, you should be able to just install Ubuntu. It will tell you how much is available to install Ubuntu on.
If you rather clear some space first, get a (re)Partitioning software for XP and make the XP partition about 10-15 Gb big (or any size above that). The rest you should define as "free space".
Then just put in the Ubuntu CD, and let the installation lead you.