Reformatting the drive is generally enough to wipe it clean (I believe it wipes everything except the boot sector, but I'm not sure), but if you want to be as sure as you can that it won't come back, you can use a program like dban
(please note that dban will wipe the information off of every hard drive within the computer).
You should be able to use any XP home edition cd to run the reinstall so long as you use the cd key that originally came with the computer. If you can, just borrow a friend's disk (make sure it's XP home edition or else your key won't work with it) and use that. If none of your friends have a disk, you can always try calling Dell to see if they can send you a replacement disk (might still cost some money, but not as much as buying a retail version). If you end up just buying another disk from a local store, you're probably going to be looking at a price tag of about $200.
Also, you might want to get all the drivers for your computer saved to a flash drive or cd before running the reformat and reinstall to speed up the process (go to this page
and follow the prompts to find your computer and download the needed drivers; if you get replacement disks from Dell, you might not have to worry about this).
Another thing: be sure to install security software (firewall and antivirus software) before connecting to the internet. Just download the install files on another computer and save them to a cd or flash drive. Once windows has been reinstalled, you can copy and paste the install files onto the computer and run them.