Sorry Ian, getting the names mixed up here. Had Video Factory, bought Screenblast (CD), paid for the download upgrade to Vegas Movie Studio + DVD. Now sitting here trying to find out how I get the download back without paying for it again. Still have the CD for Screenblast sitting here.
If you purchased these on CD then the serial number is inside the front cover of the install/getting started booklet. If you purchased them as downloads then the serial number was emailed to you, and hopefully you've printed that or otherwise saved it in a safe place. If you don't have your serial number anymore then contact customer service and give them what information you can about the purchase, such as your name, email address, phone number, etc. that you used when you originally registered it. They'll be able to give you a new serial number free of charge.
I don't remember them emailing me a serial number, I think instead that I had to put in the serial number of my CD copy of Screenblast, to prove that I qualified for Vegas Studio + DVD at the reduced price. If they emailed me a serial number, I put it in a very safe place: somewhere on the HD that got reformatted. (insert sound of hand smacking forehead).