It's probably all been asked before...

Which country are you going to?

The category says CR BOOMER1. I go for long periods of time and never consider it. I do use lots of deet though

The chances of contracting malaria on the northern Pacific coast of Costa Rica are fantastically small and to undergo a course of anti-malarials i.e. stuff your body full of strong drugs for an extended period before and after your 1 week vacation would in my opinion be madness.
That said- Dengue Fever is now quite endemic during the rainy season. Santa Cruz notched up more reported cases last year than anywhere in CA and in Tamarindo virtually every resident seemed to either be suffering or know someone who was.
There are no drugs to prevent dengue, so make sure you use appropriate repellants and remember the dengue mossie is a daytime beastie too.

If anything...I might consider taking Dukoral which is an oral vaccine to stop travelers diarrhea.

Malaria in Costa Rica is rare, few tourist ever get it, its mainly in southern zone, in the canopy, well away from anywhere most travelers venture too...

The NW is least likely, you should be more concerned with Dungue fever outbreaks, and car accidents, then snakes.

Best way to make this decision is to ask what the hotel workers, business owners, etc. in the area you will be traveling are using. Basically, none of us in Costa Rica use antimalarials. Partly because the medication produces horrible side effects, partly because we live away from squalor.
Malaria is generally considered an "occupational hazard" by CDC -- that translates into poor sanitation areas, places where drinking water, sewer etc. are open. The "canopy" is not one of these areas, and the rain forest is perhaps the safest place to be from the nasty diseases.
Dengue is a similar life cycle, and also similar in who most often gets affected. However, there is no vaccine. Just be aware when returning to USA that, should you come down with a high fever (nickname is "bone-breaking illness, because your bones hurt) your doctor will amost immediately assume malaria, rather than dengue. So if you do come down with symptoms, don't always assume the Doctor is Right -- often it is just a flu you picked up on the airplane flight (notorious source of illnesses).

Just to clarify I am one of those people who, while trying to be cautious, does not like to take any kind of medication. I'd like to stop at hep. injections though have been strongly advised to get typhoid and tetanus/diptheria shots as well. No doubt I'd be innoculated to the eyeballs and suffer a cold the entire week.
Ticotim thanks for the warm thoughts of being hit by a car or being bitten by a viper. Am much less worried about contracting malaria now. We will be coming from Canada where the mosquitoes are horrendous (very rainy in June), and cases of West Nile virus also for which there is no vaccine. Perhaps we shall swathe ourselves in a potato sack soaked in repellant. It's a thought anyway. With dengue and malaria is it those who are weakened and children who are most prone? Here in Canada scientists say it is the young, the elderly and those with supressed immune systems who are at risk for the west nile virus. Sure glad I'm not paranoid.

Sky

OP i think the July edition of National geographic has a big article on malaria. Well worth reading to understand it.

also anti malarial drugs just treat symptoms not stop you from getting malaria. I'd have travel insurance though.....

You do not need to be in any way paranoid. You will not get malaria and if you consider some light deet protection then you will not get dengue either. Or anything else.
Enjoy.
(I have lived here for more than three years, traveled all over in all seasons with no medications at all and had nothing worse than a scorpion bite. Hurt though).