Chuck--It's good to ask this ahead of time!! My husband and I drove from Barajas to Avila our first day there last year, and got horribly lost (we took a wrong turn off the highway and ended up in downtown Madrid during morning rush hour).
The highway from the airport is a bit confusing, with lots of dedicated lanes for exits--so be sure to have good directions and follow signs carefully! In an unfamiliar rental car, it's hard to be as aggressive as you need to be to change lanes on a crowded highway.
In fact, if you arrive in the morning, it might help to wait at the airport (or isn't there some fantastic churros place in Barajas town?) until the worst of morning rush is over.
I'm sure someone here can give you perfect directions, lane changes and all!
Tara
P.S. Info about churros in Barajas at
http://www.madridman.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic&f=5&t=000158&p=