Right click on the file you want to open, go down to "Open With," then select Windows Picture and Fax viewer. If its not on the list, go down to "Choose Program..." and it should be on the list of recommended programs. If you want Windows Picture and Fax Viewer to always open that type of file, click on the "Always use selected program to open this kind of file" option, then hit Ok.
Edit: In Vista (and possibly Windows 7) they renamed it to "Windows Photo Gallery." Otherwise, the process is the same.
I believe it's built into the OS, so as long as it's a Windows platform, it should have the program installed.