need to delete my pc, going to buy a new one and sell my old pc

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pleas I need help, I need to delete my pc and leave it like the first day I
buyed it.
I know theres in the comand prompt, I've used it in the past and sucesfully
deleted. Was about3 years ago and can't remember the comands. System restore
just go back 1 month, so I can't use it. Tha laptop have 2 partitions and the
windows xp it's loaded in the computer, I don't have a disck, so if I delete
all the system keep the xp and then I can install it again and have it ready
for sale, I dont want my personal info in others hands. My pc is an Acer
Aspire 5670.Dont worry I allready made a backup in an external hdd, so I have
nothing to loose. please help and tell me the commands to do a full delete.
Ah, I try formating the C: but don't allow me.
.