The SetPriority method attempts to change the execution priority of the process. In order to set the priority to Realtime, ...

The SetPriority method attempts to change the execution priority of the process. In order to set the priority to Realtime, the caller must hold the SeIncreaseBasePriorityPrivilege.  Without this privilege, the highest the priority can be set to is High priority.
The method returns an integer value that can be interpretted as follows: 
0 - Successful completion.
2 - The user does not have access to the requested information.
3 - The user does not have sufficient privilge.
8 - Unknown failure.
9 - The path specified does not exist.
21 - The specified parameter is invalid.
Other - For integer values other than those listed above, refer to Win32 error code documentation.