If you ran Factory Recovery from the disks as it seems from your last comment, then run it again from it's hotkey at boot:
Dell Restoring Your Computer´s Software to the Factory Settings
This is a more stable method which is best to try first, saving the disks as a back up or if you need to replace the HD.
If both methods fail when tried again, you'll either need to order new Recovery disks or a Reinstallation DVD from Dell. The Dell DVD is the same as the retail Windows 7 installation DVD without the factory bloatware but also without auto-activation.
However if you can still read the Product key on COA sticker then you can use the latest official Windows 7 installation DVD with SP1 downloaded from here:
Win7 Home Premium x64 SP1 English Official ISO download.
Burn to DVD or write to flash stick using
Windows 7 USB-DVD Download Too.
Follow these steps to get a perfect clean reinstall of factory OEM:
Reinstalling Windows 7