Installing Windows would not have any effect on the address book. It's a webpage, not a program on your computer. Try signing out of the account and back in again. Are you using only the webmail with a browser, or are you using a desktop client like Thunderbird or Outlook?
If you use GOOGLE Chrome, Follow these instructions and you should be all set !
Reset the Google Chrome Web Browser to Default Settings : http://customer.xfinity.com/help-and-support/internet/reset-google-chrome-browser/