Open your pictures and click on Tools at the top. If you have Vista you may have to click on Organize then Menu bar then Tools you can set your preferences in there and then click on Apply to all folders if that is what you want and then OK...
That should do it for you.
I too think it is a pain not to have the previews all the time but sometimes I need to just look up a file number and then I have to change it back so I am so used to changing it everytime that it don't bother me now, but it sure did at first.. I think what bugs me more is that I now have to choose the size of the thumbnail I want.....lol.....
Hope that helps....
BTW don't be afraid to open up something and take a look at it .....if you need to know what something is don't be afraid to ask......