Hi SK! How's everything? If you 'right click' on an image file, you will find and option to set the default program to open it with. Yes, if you uninstall the program the settings should default to the original program settings.
Which version of Vista is on your system? If it is either Home Premium or Ultimate, you can launch Windows Media Center which will provide you with a great photo viewer as well as other multimedia options.
Most of the free viewers such as irfanview, picassa, xnview and others, Adobe also has a free image viewer and editor, also do a good job. Just don't start installing too many of them as sometimes it can cause conflicts as each will want to be the default viewer and sometimes you end up with one program opening jpg, another opening gif, and so on... The programs will give you a choice on whether to make it the default viewer or not during installation so don't forget to read the screens when you are installing. And be aware also for programs that try to install various toolbars or other 'free' programs that you don't really want. You may find yourself with 4 or 5 extra and unwanted toolbars when you open your browser, not to mention spyware junk.