Well, this is a hard one to answer...
If you have a problem that involves just switching a card or something, AW will send you the part, and assist you over the phone to install it.
If it's something bigger than that, like a power supply or something, AW can arrange for a local technician to visit your home to do the work.
Usually for major problems, like reboots or blue screens that can't be solved over the phone, AW will pay to have the computer shipped back to the depot where they will repair it and go through the run in process again. They do this more now as they've had problems with technicians not doing quality work.