It might help if you exsplain WHY you want to do a factor Restore.
Is it because you have a problem.
If No problems then use the method I outlined.
If you Have a problem, then that will not work and mostly likely you will need a Boot disk that will access the the restore Partiton, or look for a "Restore program inside the restore partition.
You can not restore the "C drive" when it is the active operating system.
The Manual should state how to do a restore, if you do not have the manual, go to Dell's support website and download, read, then ask questions if unsure.