Hi Frede,
1. Are you referring to GPS Pathfinder Office?
2. What happens when you open Pathfinder? Do you receive any errors?
3. Are you using a 3^rd^ party antivirus application on your computer?
If you are referring to GPS Pathfinder Office, it is compatible with Windows 8.1. You may check this information in the Windows Compatibility Center.
If you are using any third party antivirus, temporarily disable it and then check if the Pathfinder works fine.
You may refer to the antivirus maker website to check for the options to disable antivirus.
Warning: Security software can help protect your computer against viruses and other security threats. In most cases, you shouldn't disable your antivirus software. If you have to temporarily disable it to install other software, you should re-enable it as soon as you're done. If you're connected to the Internet or a network while your antivirus software is disabled, your computer is vulnerable to attacks.
Hope this helps and do let us know if you need any assistance.