You may need to re-associate the file types with the program. How you do this depends on your OS. In XP and from Explorer select 'Tools', Folder Options', 'File Type'. Navigate to the 'Tif' filetype and click the 'Change' button then select the program you wish to open this type of file. You mave to do this for other file types e.g. 'bmp', 'jpg' etc.
I have totally forgotten how to do this with other versions of Windows so if you're not using XP hopefully someone else can help.