Concerning Windows XP: Doing a SEARCH for images (*.jpg, *.bmp, ...) and switching to thumbnail-view does only TEMPORARILY generate those thumbnails: NO thumbs.db file(s) get generated and stored!
But recently I found out a special trick that can be used for an automatization procedure (e.g. command batch-file): If you first copy a (wrong) dummy thumbs.db-file into a folder and open that directory with windows explorer in thumbnail-view, you now DON´T HAVE TO SCROLL through the directory any more to get all thumbs generated!
Certainly it will take some time, especially in huge image-folders, until your (slow) single-core PC finishes the generation of the thumbs.db file. When it´s generated, or doesn´t grow in size any more, you can recursively traverse to the next directory in your batch-file.