This is something you can check if you have Win98. I'm not sure how it works in other operating systems. There may be something similar.

Pick one file type and double check the folder options in Windows explorer. On the "file types" tab locate the file type in question and see if the "open" option is set to the correct program. If it isn't set it to the correct program and do that for each file type.

If that doesn't do it I'm out of ideas.
Good luck