The partitions as installed by Dell would be: Dell Media Direct, Onboard Diagnostics, Windows and Image Restore (the restore image). The onboard diags partition is useful, but the Dell Resource (aka Drivers & Utilities) CD has the same diags if you boot from it. The Image Restore partition is only going to work if the MBR is unchanged from (or repaired back to) how Dell set up the drive. If you don't want to reload the out of box factory image then removing it gives you back a few gigabytes of space.
As for the system after reinstalling XP, it sounds like you have installed the software from the Resource CD, but haven't actually installed the required drivers. The list of XP drivers, in the order they should be installed, for the 6400/E1505 is: