What type of nail or fastener should i use before the carpet goes down?

Quickest way is to use ring shanked nails in a power nailer. Check your local
tool rental if you don't have one. Sounds like the floor has no other future,
so even if you did hardwood later, you'd want the old flooring tight so you
could put down the new stuff over it. (Power nailers are more fun than paint
ball guns...Pow!...Pow!...Pow! Everyone will be impressed.)
HTH

Joe