I think you have done the avast installation several times in the same folder.
To solve this, uninstall it from control panel, create a new folder in your program files(i.e. Avast) and re-install your product.
If the unsinstallation is not successful through the control panel, restart your PC, press F8, select safe mode, and delete the content of the avast installation folder.
NB one file will refuse to be deleted.