Help on reformatting hard drive (C)

hello there i have a dimension 9100 windows xp, im trying to wipe the hole computer and start again. i can get windows to reinstall but once that is done i still have the old bits on my c drive. i wanna clear/wipe my drive as i have a virus on it. i have the cd back up that came with the computer and put that in and it starts up in black screen i have two options start from disk or hard drive, im runniung from disk then it gives me more options in what i want to do but there is nothing saying about reformatting c drive, i have reinstall windows and it works fine but then it juts copys all my old files from my c dirve again which i dont want. i just need to no how to wipe my c drive but keep all the inportant files on it that makes my computer work, i have spoken to dell before and they gave me a code to do it in the dos screen but i cant remember it now, im just seeing if any one can help me i just wanna wipe the hole compputer and start again.