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:
Dell-Larry R is right on with his suggestion. I might just clarify a item or two. You can delete all the partitions on the hard drive except for the first partition, which is hidden. It is only about 50-54 MBs. The 😧 partition is the Restore partition. Since you mention you have already deleted it, you cannot do a Dell Recovery Restore. That is the Recovery partition. When installing the OS, you can make the OS prtition (C) as large as it will go. This would insure that you are using all the hard drive capacity except for the first hidden partition. This can be removed also, but it is a rather complicated task that has to be done from the command line. I had a Dell 1505 and ran into this same situation and removed the hidden partition and created a single partition for the OS. You then will have to have downloaded all the drivers for the system as Dell-Larry R mentioned, then install them and all the programs you want, go to MS and downlaod and install all the updates and you should be fine. I did it and it worked well. Hope this helps as an addition to Dell-Larry R's instructions.