Hi, Carole-
If you're renting from Avis, I can tell you where the drop-off on Gran Via is...it will be on your right as you go from the Cibeles fountain toward Puerta del Sol, four or so blocks past Calles Hortaleza and Fuencarral (which you'll also see on your right). Once you see the little storefront (not very well marked if you're going fast), turn RIGHT onto the next street and go into the underground parking garage.
You take a ticket and go to the second level underground, and there is someone to check you in (you have to go back up to street level to finish up at the office on Gran Via, though). We had NO IDEA where we were going...ended up double parked while I ran into the Avis office...cut off a bunch of taxis...so I hope this saves you a little headache.
Oh, and as for driving in Madrid, it's horrid! Just take it easy and try to have your directions in your head so that you don't have to try to navigate too much!
Tara
[This message has been edited by MadridMan (edited 03-28-2001).]