If you have the Ubuntu CD then you are ahead if you dont go download it from Ubutnu's website
Once you get the cd please do the following:
1. Insert the disk into the CD drive and reboot
2. Just before the Anything start loading press F12 which will open a menu to boot from Hard Drive or CD, Choose to boot From CD.
3. Then it will boot into a graphical menu..You click Install Ubuntu #1 Option on there.
4. Ubuntu will load itself on your RAM and starts booting
5. when ubuntu is done loading ( when all the icons are on the desktop) you then click a icon called install , it will open a Install Wizard to install Ubuntu,
5. Answer all the questions if there are any until you get to the Partition Part
when that happens click on Manual partion.
6. that will show you all the partions on your computer
7. Shrink the Windows Partition to how small you want it, Shrink it so Ubuntu will have 15Gb to be installed on.
8. after you shrink the windows partition you then , Make a New partion called
Swap make it about 512mb, after that make a 15gb partion from the resize you did and set it as /
8. click next.
9. the disk will then be formatted
10. keep clicking next until you get to the install screen.
11. When you get there click install and ubuntu will start to installon your 15gb partion
12. after the installation is over grub will popup and you configure the dual boot.
13. then you restart.
14. the grub menu will appear with it showing to boot from XP Pro or Linux
Choose any you ared done.
If you dont understand this than I dont know how to explain it.
I dont know english that well and i was kinda in a hurry.. Please search for Articles on how to dual boot ubuntu by searching google...