Accidentally putting a tick or overlooking the tick present by default in the option "Always use the selected program to open this kind of file" causes a file to open with a selected program when you may not want it to be that way all the future. How do you undo it?
1. Click Start -> Settings -> Control Panel -> Folder Options.
2. Goto "File Types" Tab
3. Scroll down to the file extension type that was mistakenly set to "Always use the selected....."
4. Click "Restore" button. The file association will change to default and "Restore" button will change to "Advanced"
If you want the file to always open by default with any other program of your choice the in Step 4, click "Change" button. Select the program of your choice and make sure ""Always use the selected program to open this kind of file" is checked.