What version of Ubuntu are you trying to install?
I dual boot Ubuntu with XP too.
What I simply did was to partition my HDD and create a 10gb partition for Ubuntu and a 400mb for the swap partition.
After you do this, when installing Ubuntu, choose the option of manually partition and install, then select the partition you have already created (10 gb in my own case) as ur "/" boot partition (the format should be ext 3) and the oda (400mb in my case) as swap.
After installation, Ubuntu will automatically instal grub, from where you can choose the operating system you want to boot into.
I hope this is helpful.