In the Samsung Kies application under "Tools" there is "Firmware Upgrade and Initialisation,"
With your device unplugged (it will say not available for this model if its plugged in, don't plug it in until it tells you to when in download mode) select this and run the what it tells you to do,
After this is finished you will have a completely fresh install of the latest officially released firmware (for your original build eg. carrier branded) with original serial number and imei number
This does not work for all but its worth a try, you have nothing to lose