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I don't own a Dell but I do have a HP laptop. Did you delete the restore partition of you laptop if it came with one? If not then use that to restore to factory defaults.
Or if you have the correct Vista install disc, you could do a fresh install, using the code from your Windows licence sticker. You have to toss up whether you are prepared to get in cleaner install, without McAfee, etc with the possiblity of having to install drivers. If you don't really know what's involved, I would suggest recovery. Look for Start Menu entries for Dell recovery, or google a guide for your specific computer - some approach recovery in different ways.