Are your custom tunes on the broken PC or did you save them on a separate storage device? I cant remember if/when you update the SCT if it clears out any custom tunes that are stored on the device!
If you have the custom tunes saved to a separate storage device, You need to return the truck to stock first, you must return to stock first when you are about to update the firmware from SCT. Go Back To Stock Before Updating Your SCT
Then on the new PC, load up the software and drivers from SCT web site. You will now be able to load up the stored custom tunes to your PC then load the SCT device. Once the SCT device has the tunes on it, you can now load up tune to truck in order to pull the stock file. Once the stock file is back on the SCT device, you can go back to the PC and save the stock file.
How To Save Your Stock File With An SCT
I had to do this when I got a new PC, I now have my stock and custom tunes on 2 PC's. I would also suggest that you buy a 1 or 2 gig flash drive to save you stock and custom tunes on, you never know when your PC will take a dump or if the tunes get accidentally deleted. You should also never rely on saving the custom tunes on your email.