You can system restore by clicking start/home then in the search box type in "system restore" and hit enter an click system restore then follow the instructions from there. Just select which date you would like to restore it back to then wait.
Going by the title of your thread,by system restore,i think you mean restoring PC to its original state.
To do this you need to do a clean install with your OS disk.
that could be done if he ment factory settings or you could still do it the system restore way by going to the earliest date possible which is normally the date you had first turned on the pc or basically when pcs sent out the pc to sintect, unless this option was disable?
Old System restore points are deleted or overwritten,how often this happens depends on how much space you have allocated in system protection settings.
System restore is a bit of a hit or a miss & will not restore your system, but a full backup will. System Restore only restores select portions of the system, such as the registry, for example.