Many software programs do this, some allow you to override this behavior by choosing custom install options but most of us generally just take the defaults. Norton System works used to have a program that would follow installs and allow you to undo more than just what the uninstaller did. I don't know if it's in the current incarnation.But there were other forces at work. My oldest granddaughter has been using Adobe Photo (?) in her high school photograph class. The wife was impressed with the product and bought us a less expensive version of it, it is titled Adobe Elements. I loaded it on my computer and re-claimed my photos from the Dell software that came with the new printer and was installed on the new laptop. I hate it, I have un-installed both Jasc Paint Shop Photo album, and Jasc Paint Shop Pro 8 Dell Edition. I was a very unhappy boy with this photo software.
I did have a back up plan. I had stored most of photo files on a recordable CD's. I suggest that you might want to have a backup plan for photograph's stored on your computer. You do not want to lose those hunting and fishing photos. Things happen to computers. But when software modifies your files in a manner in which you did not plan I consider it a violation of trust. Something like being infected by a worm or virus.
You can change what program gets activated by what program extensions in Windows. That is, if you don't like that Adobe opens files with .jpg extentions for instance. In XP you can do it by going to: Start -> My Computer -> Tools -> Folder Options -> File Types.
If you decide to try IrfanView it will prompt you for what file types you want associated with it. It also has a neat check box menu to allow you to change them later without going into My Computer.