When you go to setup Windows XP, there will probably be 2 or 3 partitions that it reconizes. The first will be the small C:, 2nd D:, and 3rd possibly unpartitioned space. You want to highlight the two current partitions and delete them so the only option listed is unpartitioned space. If you have an 80 GB hard drive, the drive will be seen as 70 some gigs. Every drive is like this as the very first sectors of the drive contain information such as boot loader and contains information about the drive. This cannot be changed.
Of course if you delete the partitions like I said and setup a new windows install on just the 1 partition, you will loose all of your data and information currently on your system. Make sure you have your dell device drivers disk before you start otherwise finding drivers for your computer might be a hassle.
Once you do the above steps, you will have only a c: drive containing windows xp and probably around 70 gb of free space left.