You seem to be savy enough to install Ubuntu if you are using gparted to set up partitions!
The two hidden partitions probably are hidden partitions that Dell has for their utilities - there is one on my Dell, not two.
IF you created two empty partitions with Gparted, you should have 6 partitions (from my reading of what you wrote): C:, D:, two hidden partitions, and two unformatted partitions created under Gparted.
Use gparted to see how linux identifies the partitions you created - hdx, hdy, or sdx, sdy (x & y being numbers). gparted should show them either as formatted or unformatted, depending on what you did.) I would create 3 partitions available, not 2 - sorry.
Assuming space is not an issue, I would set up a "swap" partition - 2 times your RAM, a root partition "/" of around 5 gigs, and a "/home" partition of around 5 gigs as well. (/home makes updates and things easier - your settings and data are stored there.) You do need a minimum of 2 partitions (swap and / - swap is max 2 times your RAM, bigger is a waste - and you could easily use less for swap).
When you have the partitions set up (you can do this within Ubuntu as well, but you seem to have gparted under control, so I think it is safer this way). Write down the partitions and how you are going to use them:
swap for example is /dev/sdx
/ for example is /dev/sdy and ext3 format
/home for example is /dev/sdz and ext3 format
When everything is ready, fire up the Ubuntu liveCD, get to the desktop, click on the install to harddisk icon, and walk through the process. When you get to the partitioning part (3rd or 4th step, I forget) choose manual partitioning, and set it up using the partition information you wrote down.
It should go relatively smoothly - barring unforeseen problems.
Make sure you have your XP and other software installation CDs and serial numbers in hand BEFORE you start. Make sure you backup your data BEFORE you start. And read each screen carefully before you click on next.
The installation process is going to prompt you to install grub - on the MBR. It should correctly identify and set up the bootloader to include both the Ubuntu (as default) and XP.
When the install is done, reboot, start up XP first to check that it is ok, then reboot and start Linuxing.
If you run into problems, use gparted to make sure you didn't accidently overwrite your windoze partitions, and running fixmbr from your windoze install CD will restore the windoze bootloader.
One last point, Ubuntu can read and write to NTFS partitions.