system
5
There may be no infection…
Since it was the Web Shield that caught the error, were you/your friend surfing the web when the error message occurred, or did it pop up randomly when no one was online?
Are you using the Avast Online Security browser plugin?
There is a link here in the forum for the logs you need to provide if you want someone to do a deep dive of your system for malware…
http://forum.avast.com/index.php?topic=53253.0