I bought the regular version of Prism (or InStat), but the demo version that I had before runs instead.

Both the demo and the full program (for Windows) are named PRISM.EXE. If you install the full program using the default settings--placing the full program in the same folder as the demo version--the demo is overwritten. But if you install the two into different folders, you will have both versions on your computer, and when you launch Prism using an icon, you'll start the version that the icon points to.

If you can find the demo folder, delete it and all icons that point to it (to see where an icon "points"on a PC, right-click, then choose "Properties"). If you can't tell which is which, search for all copies of PRISM.EXE and delete all folders that contain those copies, making sure there are no data files you want to keep. Then install Prism again.