This is the "official" Dell reinstallation procedure. Maybe you were not following that to the letter.
Burn all your important files to a CD or store on a memory stick. Remember you will lose each bit of information that you had stored in your computer including the programs that you had installed, such as Microsoft Office.
Unplug all the devices and drivers from your CPU (except the keyboard and the mouse), including your printer, camera and memory stick.
Shut off the computer and then turn it on, or simply restart it.
As soon as you see the Dell logo at the start keep on hitting the F8 key repeatedly. Do not stop until you see the screen that says "Repair your computer." There will be other options too but chose this option and hit Enter.
Next you will see a prompt window giving you options to select the keyboard layout select from the drop-down menu "US" and click "OK."
On the next screen you will be asked to select the user ID and the password of your Windows Vista account. Select your user account, but make sure your user account has administrative functions. Type in your password. If you don't have a password, leave it blank and click "OK."
Now you will come to a window with multiple options. Choose the one at the bottom that reads, "Dell Factory Image Restore." If you are unsure at this point whether you want to go ahead and restore your factory settings you may click "Restart" or the "Shut Down" buttons at the bottom of this prompt window. The computer will restart your computer without executing this operation, and you will get your most recent settings back. Otherwise clicking "Dell Factory Image Restore" will generate an operation that will take about 45 minutes to complete.