It is possible that the site might have been hacked, what is the URL change the http to hXXp to break any active link to avoid accidental exposure.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
Vista, win7 - Repair of avast. Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
You may need to reboot after the repair.
Using windows notepad, check the anti-rootkit scan log this should hopefully contain details of the detection - For winXP C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\log\aswAr.log. For Vista/win7 C:\ProgramData\Alwil Software\Avast5\log\aswAr.log.