Cinderella,
Unless you know what you're doing I would suggest extreme caution. If it's a really old photo you may damage it beyond repair taking it out of the frame. Here's a good thread that you might look at.
http://www.retouchpro.com/forums/sho...ight=Ambrotype
I'm not sure it is an ambrotype or daguerreotype you're talking about but if you need more info check out search on the Retouch Pro menu and search out more on these two types of photos. There's lots of good info.
All in all I agree with the suggestions others have given you. Check it out by a pro first because damage is permanent. Good luck and let us know what you find out. If it does come out of the frame and you want to scan it, you will probably have to scan it in sections.
DJ