In NTFS volume, only the owner of an object (file, folder, ...) that can change permissions of that object. You need to take ownership of your drive C: somehow, after that only you can grant permissions to users.
Try to enter safe mode, login as "Administrator" (enable that account first if it is currently disabled). Then right click C: drive, click Properties. Navigate to Security Tab, then click Advanced. Go to Owner tab, then change the ownership to "Administrator" first. Then, grant permissions like these to these users [CMIIW]:
Authenticated Users: Read, execute, write, modify,
Administrators: Full Control
TrustedInstaller: Full Control
SYSTEM: Full Control
Users: Read, execute
Finally, change the owner once more to "TrustedInstaller".
I have tried this, but i keep getting errors such as
"An error occurred while applying security information to:
C:Boot
Failed to enumerate objects in the container. Access is denied.
An error occurred while applying security information to:
C:ootmgr
Failed to enumerate objects in the container. Access is denied.
An error occurred while applying security information to:
C:hiberfil.sys
Failed to enumerate objects in the container. Access is denied."