When I first downloaded picasa 3.0, and looked at the new features, I must admit I thought the picasa photo viewer was a pretty nifty, fast and easy and it "transparently" overlayed the view of the image over the working window, rather than opening yet another window frame. It also creates and iphoto style film strip under the image and it gives you access to a number of picasa actions like zooming in uploading to the web & emailing without having to fire up picasa.
Well I've come to the conclusion I'd rather fire up picasa to do things with my photos and just want explorer to handle images as before. Now the pesky problem of how to restore the default viewer for all those graphic file types. It turned out to easy. Under tools menu in Picasa select the item Configure Photo Viewer an you will see the dialogue (show on the left), and tick the don't use Picasa Photo Viewer line and it will not appear and you will be back to your previous setting. If I change my mind I just have to visit tools/configure photo viewer again and change my selection