Windows XP's built-in image browser is really fantastic. It has (almost) completely replaced Irfanview for me. The thumbnails are stored in a database, so once you have generated them, they load almost instantly. You can change filenames and directory names, choose a picture to edit, view slideshows or filmstrip style. Best of all, once you've rotated an image, it remains in that orientation the next time you view it! That has to be the best feature - saves a lot of neckache.