I was also having this problem of how to download models from 3D Warehouse running Windows XP and SketchUp version 8. This is how I resolved the problem.
In 3D warehouse window showing the model tiles, click on the model image itself rather than the download icon in the lower right of the tile. A larger view of the model will open. Left click the large red download button in the upper right corner and a drop down menu will give you the option to download the model in whichever version you require (versions 15, 14, 13, and 8).
Save the file to a folder on your local drive then either:
open the file in a separate SketchUp window, cut and paste the model in to where you want it,
or drag and drop from file folder into SketchUp drawing window.