How to restore a Dell Inspiron 1545 to factory settings without the disk?

 Hi, My name is Samyak Veera from U.S

I want to restore my Dell laptop to factory settings without the disk. People are telling to press F8 when I turn on my computer and go under repair  and such but every time I do that it says I need the disk so I have no idea how I can do this...Any ideas would be great thanks! Once again it's a Dell Inspiron 1545.

Thanks

Samyak Veera