I will need to get from Port Washinhton to LaGuardia on a Wednesday afternoon. Do I need to arrange taxi or limo in advance, and do any of you have recommendations for which company. I do not want to take public transportation. I do know that a taxi or limo will cost more. I will need to get to LGA quickly.

I don't know of companies, but if you have to be at LGA at a certain time you should definitely reserve in advance. Here's one I found but I don't know anything about them:
Delux Transportation Services
www.deluxtransportation.com

Thank you dianedancer. Much appreciated.

I would arrange a car service in advance since on LI taxis may not always be available on a moment's notice (I have waited as long as 30 minutes from when I called). Try Carmel or Dial 7 - and look on their web sites for coupons before reserving the car.

Absolutely take a "limo". Port Washington must have airport car service or any one of those upscale North Shore communities. Google car services to apts.

Deluxe has served well. Be sure to add extra time for the inevitable traffic jam.