When you purchased that 2009 PC from Dell, did it come with the Windows VISTA install CD? If so use that CD to do a clean Windows VISTA install. If you do not have the CD, contact Dell and they will probably send it to you for a few $.
Hint: When you purchase a new computer, ALWAYS, immediately contact the company (Dell,HP, etc.) and tell them you want a copy of the operating system on CD or on a USB stick. They will gladly provide. Sometimes they will provide for free and sometimes they want say $15.95, etc.. Having the OS on CD or on a USB stick is a neck saver down the road when faced with situations like you are in now.