Look, mine is on drive E:

It won’t harm… If that does not work, follow:

  1. Uninstall avast from Control Panel first.
  2. Boot
  3. Use Avast Uninstall for complete uninstallation (read http://www.avast.com/eng/avast_uninstall_util.html)
  4. Boot
  5. Install again
  6. Boot