Problem: When I double click on a Prism file in Windows, I see the message "To open this file, you must install GraphPad Prism or the Prism viewer."

When you double-click on a file, Windows looks at the extension and tries to find the corresponding application that opens that kind of file. So Prism has to register the .pzf and .pzfx file types with Windows. Every time Prism launches, it checks that that the registration is correct. If it isn't, Prism sends the correct commands to Windows to create or fix the registration. But... Prism can only do so if it is running with administrative privileges. If it doesn't have admin rights, Prism isnt't allowed to create or fix the registration of the file types. If you double click on a Prism file and the registration is not in place, Windows won't know it needs to launch Prism. 

The fix is simple: You need to run Prism once while it has admin rights. The simplest way to do this is to right-click on the Prism icon, and then choose from the shortcut menu, "Run as administrator".

Notes:

• Some institutions lock down admin rights, so you'll need to get help from IT to give Prism admin rights. 
• Double clicking on a file is only one way to open it. The alternative is to launch Prism itself, and then open the Prism file from the Welcome dialog or using the File..Open.. dialog. 
• If Prism hasn't registered its extensions with Windows, why does the message mention GraphPad Prism? Because somehow, those file types got registered to Windows Desktop Search, which uses an add-on we created so it can index Prism files.