The shutdown last week of a U.S.-based Web hosting company crippled more than 500,000 bots, or compromised computers, which no longer are able to receive commands from criminals, a security researcher said Tuesday.

Although the infected PCs are still operational, the previously-planted malware that tells them what to do cannot receive instructions because of the shutdown last week of McColo.,Half a million bots are either offline or not communicating" with their command-and-control servers, estimated Joe Stewart , director of malware research at SecureWorks Inc.

Malicious Software Removal Tool (MSRT), purged nearly 300,000 infected PCs of the infamous Storm Trojan.