John,
My, guess is that you need a belt and not a pulley. Pop open your hood and inspect the serpentine belt. See if it looks frayed, worn or even if it has a "cooked" kind of look to it (likely caused by the slipping). If it does, all you need to do is release the belt tensioner, slip off the old belt and put on the new one! The unit on the bottom left of [http://image.4wheeloffroad.com/f/9300259+w750+st0/131_0610_01_z+2007_hummer_h3+engine.jpg|this picture] is the tensioner.
As a side note, you may find that the tensioner was loose for some reason. If the tensioner is loose, I would recommend replacing the belt anyways, as it may have been damaged.