How do I remove the default program?

I have windows vista 32-bit and I have a .dat file that I had to open with wordpad one time to change information in it and I left the always open this program check and the icon was to change to a musical note and now the file always open in wordpad how change I reset it back to not having any default programs I tried everything and I am going to delete all the information but will that work?|||There is no default program for dat files. Dat files are extensions used in tons of different programs. All you did was assign a program to view it. You didnt change anything. the program it is associated with will still see it as it always has. I%26#039;d suggest you leave it alone