1.) Install the software that is in the Setup folder 
2.) When prompted for a serial number, install the Freeware Edition. 
3.) Once the installation is complete close the process "PRTG Server.exe" (CTRL + ALT + DEL, Processes tab) 
4.) Stop the two services "PRTG Core Server" and "PRTG Probe Service" (menu Start, Run, services.msc) 
4) Copy the crack file contained in the installation folder by default. "C: \ Program Files (x86) \ PRTG Network Monitor" 
5.) Change the registry key (Start menu, Run, regedit.exe) 

Go to this place 
"HKEY_LOCAL_MACHINE \ SYSTEM \ ControlSet001 \ Services \ PRTGCoreService" 

Right-click the ImagePath key, and click edit. 

replace 

ImagePath = "C: \ Program Files (x86) \ PRTG Network Monitor \ 64bit \ PRTG Server.exe" 

With this: 

ImagePath = "C: \ Program Files (x86) \ PRTG Network Monitor \ PRTG Server.exe" 


6) Redémmarer computer or redémmarer both services previously stop "PRTG Core Server" and "PRTG Probe Service" 


Finish!