Sites on what the company calls its "Most Wanted" list are collected by a community reporting system. Users of the company's antivirus software opt-in to a group whose Internet travels are tracked.
The company says about 85 percent of new computer infections are picked up by Internet surfing, and many of those come from the almost 397,000 hijacked and malicious web pages its research found in May. Sites on the list include:
Some badware sites are built with criminal activity in mind. Other sites are unsuspecting victims of sophisticated programmers that create "drive-by downloads" of badware. These can redirect users, often seamlessly, to other sites where criminal activity takes place.
About a year ago, drive-by downloads called Gumblar and Martuz infected thousands of web sites around the world, which in turn affected Internet surfers. Both were linked to botnets, which in turn can affect and use thousands of computers for illegal purposes.
Avast! also warns of hackers targeting sites like Ask.com and Earthlink.net, redirecting users of those sites to look-alike pages that contain badware.
The company's most wanted list joins other computer security companies' efforts to identify badware sites. Last August Symantec published a list of what it called the dirtiest sites of summer.