It seems something tampered with the standard programs settings. You can right click and choose open with, from here you can select the program you want to open it with and set as standard for that filetype.
To manage everything in one window, click the Windows Start button, type Standard programs and press enter. I'm using the Danish version, but it translates to the same I hope.
There should be an option called "Attach a filetype or protocol to a program" where you get a list of all available filetypes and the corresponding software to utilize the filetype.
Again, something might be lost in translation as I can't switch to an English version of windows due to Home Premium not allowing me that. And I'm too cheap for the Ultimate version
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