When you bought the software did you download it from the web site? If so you should have kept the download as this is the equivalent of a CD if you bought it from a shop.
You should also have been provided with an activation key - this is often e-mailed to you and again should be kept.
With these two parts you can install the software on your new PC provided it is compatible with Vista - if not you need to buy an updated version.
If you have neither of the above you will need to download the software again and ask the supplier if they are prepared to advise you of the key you no longer have. Details of when and how you bought it will help.