What were you doing at the time of the alert ?
If browsing it is possible that this was an alert by the Web Shield (in which case it should have stopped it getting on to your system).
- Check the avast the avastUI, Real-time Shields, Web Shield, Show report file and report the file name and full location of the detection ?
Note: change the URL from http to hXXp or www to wXw, to break the link and avoid accidental exposure to suspect sites, thanks.
If you were doing an on-demand scan at the time - Check the avast the avastUI, Scan Computer section, Scan Logs and report the file name and full location of the detection ?