If I understand correctly, the machine currently runs 7 and you want to do a clean install. Clean install means wipe everything and start over. You will lose and need to re-install all of the programs that you currently have installed. You will lose all documents, music, pictures, and other data that you have on your C drive. Is this what you intend to do?
If it is, you get an XP install disk and do the full install procedure, first allowing the procedure to erase all partitions on your drive and re-create the new one. If that's not what you are after, can you explain more clearly?