Dora, what version of Windows computer are you using? Windows 10? 8? XP?
You simply need to change the file association for Word files. The files aren't being converted, the computer just thinks those files belong to Calibre. When you installed Calibre, you clicked OK at some point when you shouldn't have.
Look here for instructions on all Windows versions:
https://www.lifewire.com/how-to-chan...indows-2624477