How to remove uninstall history? (Sony Vegas Pro Problem - Refurbished Computer)?

Here's the deal, I purchased a refurbished computer, but it didn't have all of the previous files gone. Now, I'm looking for some video editing software. I want to try the free trial on Sony Vegas Pro 10, and I downloaded it. Then, when I went to open it, it says "Your Trial has Expired". You see, I've never had a trial for it. The old user must have. So, I search for the file, sure enough "Sony Vegas Pro". I go to control panel, uninstall the file, and re-install Sony Vegas Pro 10. It still says "Your Trial has Expired". From what I found online, I must have some type of uninstall "footprint" or history. How can I remove this so I can try the trial? Thanks.