Install this link from Avast
Free –
http://www.avast.com/free-antivirus-download#tab4.
Reboot after installing. Upgrade your definitions. The newest version is
5.0.677. If for some reason a lower version is installed, a button on the main GUI in red will say "Upgrade" to the right of it...just click on it and you will get the upgraded version.
Please let me know if you have any additional questions. Thank you.