Do a complete system recovery with OEM discs

Hello,
My desktop is several years old and it does run worth beans anymore. I would like to restore
it back to the way it was when I received it brand new from Dell. I have all the discs, but
I would like to know how to go about this. I have saved my files, but I want to know
how to re-install everything so that it is like the original condition it was in when I received it.
Do I need to delete all the files or will they just be written over with the original discs when
they are reinstalled?

Thanks