In an effor to help elaborate on the previous posts... You need what they call a "Single Din" radio removal tool. Two seperate pieces of metal, each shaped into a "U" shape. The sides of your factory radio have these two holes in which you press the "u" into, and pull out.
You can pick up this tool at (most conveniently) Wal-Mart.. less than $5..
Hope this helps..
Ray