Get bootable version if possible and check what serial number you have on your old Dell computer (XP Home/Pro etc.) Start computer from bootable CD, take off partitions, create them again, format and then install. At least you will be sure you got rid of all trash from HDD
Edit:
Just to add - most of Dell computers have only 1 partiton. It's good to have at least 2 of them - around 20 GB for system partiton - keep only windows and stuff like Office, Nero etc. On second partiton save all your pictures, text documents, install games, keep movies etc etc. At least next time you will get some annoying crap or accidentially delete some system files, reinstalation will not wipe out all things you never realized you have/need and gone by then