Is it a good idea to pour a pad on top of an exsting ashphalt pad that has been down for several years ?

If you are planning on building a garage on top of slab,you will need footings to support the garage structure.

Blacktop is not normally poured on a footing that can be built on.

Chuck

Is it a good idea? In my opinion it is a bad idea for the reasons Chuck noted.

In the highway building industry it is called whitetopping. It is done infrequently, and is sometimes used at intersections where asphalt pavement is rutted from cars and especially heavy trucks braking and shoving the asphalt. The following is a Federal Highway Admin. Write-up on the subject. If the existing asphalt is in bad condition you could also consider breaking up the asphalt in place and using it along with some fresh stone as a base for the concrete. But unless you use a special (probably very expensive) concrete mix that can be placed thin, aren't you going to have tie-in problems due to another 5 inch or so rise in elevation?
Whitetopping - Innovations - Highways for LIFE - FHWA (http://www.fhwa.dot.gov/hfl/innovations/whitetopping.cfm)