First create a system restore point (this step might not be necessary as you have you have your Acronis Image).
Next follow Windows Reinstallation Guide:
http://philipyip.wordpress.com/dell-community-forums/
Follow the instructions on page 55 and download the ABRBeta.exe and run it to extract it. This will make an ABR folder. Download also the Dell Windows 7 Ultimate Activation Files and place the Ultimate backup-cert and backup-key to the extracted ABR folder. Don't run the activation_restore.exe from within this folder yet. We will need to ensure that all product keys and product activations are removed entirely.
To do this follow page 58 Step 11h and remove your product key and product activation. Restart your computer. You will get some more errors about the system not being genuine as the Windows installation will have no product key.
Then run the activation_restore.exe and restart your computer. This should reset your product activation in the same manner as if you just clean installed using a Dell Windows 7 Ultimate Reinstallation DVD.
Note the activation files will only work for eligible Dell systems i.e. systems with a Dell Windows 7 COA (which matches the Reinstallation DVD which the activation files were taken from) and a Dell BIOS.
Note: I have only mentioned these as 7 Professional in this step as it was meant to be intended for downgrade rights but the step got amended and I need to update it as I have Ultimate and Home Premium activation files taken from Dell Windows Reinstallation DVDs also.