I would recommend buying a removeable Hrd Drive. Back up the really necessary files (My Documents, etc.) Format and do a clean install.
Its a matter of the time value. No matter how thorough you are, you may not get all the "bad stuff." Bear in mind that Dell PC's come out of the box with Real Player and who knows what else pre-installed.
Also, the removeable disk with a back up program will be useful for years to come.