Heyy!! No worry man! Main hoon na! No nid to upgrade or re-install office!! Dnt even format windows!!
It's like going round your face to catch your nose!!
I know what's wrong! By mistake you must have pressed "INSERT" on your keyboard.. that's why it is overwriting the next character each time.. Try pressing the "INSERT" key on your keyboard again and see if it works.. and let me know..
If even this doesn't help, then try restarting your PC.. but i think it should work fine..
Neverless..that was plan B.. plan C is to re-install office..
which is quite funny.. and plan D to format windows!!wahhahha!!
But, chill man, I'm sure plan A will work.. try it.. it must be the "INSERT" key..