If it's not a virus, there are 2 ways to do it.
1) Right click (or shift +right click, depend on window version) on the file and choose "Open With". Then select "Choose Program..." and tick the check box "Always use the selected program to open this kind of file" and choose the program that you want that file type to associate it with in the future.
2) Go to window explorer, select "tools" > "Folder Options...". Select "File Types" tab and edit which extensions that you want to associate with which program.
Absolutely and precisely correct.... and if those two do not work, it IS a virus...
All the best Brett