how to replace Windows with Ubuntu, keeping existing storage partition

If you want to get rid of Windows XP but keep your storage partition, then after making sure you don't have any documents or other important files in your Windows partition, and **backing up any really important files in the other partition in case something goes wrong**, boot from the Ubuntu CD, select Try Ubuntu, go into the GParted Partition Editor, and remove the Windows partition but not the storage partition. In Ubuntu, your partitions will not be labeled with drive letters like C: and D:, so you may want to look at the partition layout first in Disk Management in Windows (Start > Run > diskmgmt.msc).

After that, you can then double-click on Install Ubuntu on the Ubuntu live CD's desktop, and tell the installer to install alongside other partitions. Alternatively, if you like, you can partition manually. If you want to partition manually, you may as well do that in GParted before running the installer, and then you can just tell the installer what partitions to use. One way to partition manually is to create a partition of type ext2 and size about 256 MiB (you'll specify the /boot mount point when you install), followed by a partition of type linux-swap of at least 1 GiB or twice the RAM in your system whichever is bigger (you'll specify that this will be used as the system's swap space), followed by a partition of type ext4 taking up all the space between the swap partition and your remaining storage partition (you'll specify the / mount point when you install). You'll indicate you want manual partitioning when you install, but you won't have to create any partitions because you will have already created them--instead, you'll just indicate what you want each partition for. The GRUB2 boot loader should be installed to the drive's Master Boot Record (MBR) in this case (and most cases), as will probably be the default.

Please note that this is just one suggestion--if you decide you want to partition manually (or you'd rather not but for some reason the Ubuntu installer is not giving you the ability to install Ubuntu alongside the existing storage partition), then you might want to read https://help.ubuntu.com/community/HowtoPartition.

That guide provides substantial detail and illustrates the partitioning process in GParted with pictures.