to redo the computer. You don't format first. All you need to do is make sure your computer is set up to boot first from the CD drive, put the XP CD in the drive, restart the computer and follow the instructions. The program does everything for you. It is a multi-step process. Unlike Windows 98 or ME where it tells you to remove the disk at the first restart, XP must remain in the drive for a while.
Once the XP installation is complete, you will need to install the drivers and programs. They should be included on the resource/recovery disks that came with your Dell. If you don't have them, go to the Dell web site, put in your service tag number and download what you need.
I'd still recommend you download, install and run Belarc first. It identifies everything installed on your computer along with the product key numbers. I never hurts to be over prepared. If you have any doubts, read some of the posts for help from people who can't find their disks or product key numbers and can't reinstall their programs. Or they can't remember the type of hardware installed, don't have the drivers and can't get it to work. Here is a site that shows step by step instructions for installing XP. Good luck.
http://www.blackviper.com/Articles/OS/InstallXPHome/installxphome1.htm