Hi, Welcome to BC.
Looking at your problem , i offer 2 solutions.
1) Go to download.com , type in mozilla firefox and install this browser, when prompt for selecting as default, select YES.
2) You can try to reinstall but it involves typing in commands or you can go to microsoft.com , get hold of a newer version and it must be newer than the version you are using now so that it would overwrite it, download it.
Recommend to try the first method as i am pretty sure it would work after that.....