Happens to people all the time !! --
Here is a tutorial on resetting it - Just follow the instructions there under resolution - there is a small registry file that you download and run that resets the .exe file extension for you just download it - right click and select merge (this runs the file and resets the proper registry keys back properly for you !)
The file you download is simply a text file with the following line and the .reg extension so that when merged it will correct the exe file association that was changed to winrar back so is safe (you could do the same thing using regedit but if not familiar with how it is easier to use the download !!) :
Windows Registry Editor Version 5.00
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.exe\UserChoice]