InfoWorld reports that Internet users visiting popular Web sites may be unwittingly downloading malicious code that compromises their computers and sets up a relay network for a future onslaught of spam, a security services company warned Thursday. Purportedly, the attack affects only users running Windows and the Internet Explorer browser. NetSec Inc., which provides managed security services for large businesses and government agencies, began detecting suspicious traffic on several of its customers’ networks on Thursday morning, Chief Technology Officer Brent Houlahan told InfoWorld.