Symantec implemented a "reputation-based" system in their recent anti-malware products. If you download a game from this site and experience a warning or issue where the game causes a security alert, please see this article in the Symantec Knowledge base. Note that this will also affect Windows 7 and Windows 8 systems, despite the Symantec article not including those products.
http://www.symantec.com/security_response/writeup.jsp?docid=2010-051308-1854-99&tabid=2
My school computer use Symantec Endpoint Protection, and I scanned the CW3 install file, no viruses.