The proXPN uninstaller is located in C:\Program Files(x86)\proXPN\Uninstall.exe - if you're using XP, it's C:\Program Files\proXPN\Uninstall.exe.
You can also uninstall proXPN via the control panel:
1. Open run(Press Windows key + R)
2. Type appwiz.cpl then press enter
3. Look for ProXPN in the list then select/right click -> click Change/remove or uninstall