Windows Photo Gallery automatically saves changes (unless you undo them) when you move to the next picture, and there's the problem.
If you're serious about digital imaging ("fanatic about digital imaging" also works), you should treat your original digital photo (whether you shot it with a digital camera or scanned it from a traditional print, negative or slide) as a "digital negative." You should never alter your digital negative, and you should always save changes to your digital negative to a different file.
So, if Windows Photo Gallery saves crops and other changes automatically, how can you preserve your images in their original condition?
Keeping Your Originals Safe, the Hard Way
Sure, you could create a backup folder of your photos, but that's a pain in the neck (and could consume huge amounts of disk space, especially with today's 7MP and higher-resolution cameras). Besides, you probably don't want to (or need to) edit every photo.
What if you use Windows Vista Ultimate, Enterprise or Business editions? The shadow copy service (part of System Restore) or file and folder backup may have backed up a previous version (available through the Previous Versions tab on the file's properties sheet.
Keeping Your Originals Safe, the Easy Way
But, wait a minute! If you just downloaded the file from your digital camera, a previous version isn't available yet. And besides, if you're using a home edition, no previous version for you!
No matter what edition of Windows Vista you use, here's an easy way to save the changes you make with Photo Gallery without dinging your original photo:
Open the picture in Windows Photo Gallery
Open the Fix menu and make all of the changes you want (cropping, color correction, exposure settings, and so on)
Right-click the edited picture in Photo Gallery and select Copy
Use the Pictures Explorer to navigate to the location you use to store edited photos
Right-click an empty area in the folder and select Paste to save the edited picture to the location you selected in Step 4.
Return to your picture in Photo Gallery, open the Undo menu and select Undo All to remove all of your edits.
There. Now you can edit away, and you still have your original photos. It's a good feeling: a digitally good feeling.