Microsoft's Hotmail on Monday dumped McAfee and switched to Trend Micro to provide anti-virus scanning for messages sent and received by the nearly 200 million users of the Web e-mail service.

As part of the swap in anti-virus providers, Hotmail users' attachments -- both incoming and outgoing -- will also be scanned for possible malicious code, said Trend Micro and Microsoft in a joint statement.

The Cupertino, Calif.-based security firm is best known for its PC-cillin anti-virus product line.

No real explanation was given by Microsoft for the switcheroo, other than an oblique line in a statement issued Monday.

"MSN is focused on providing a safer online experience for consumers, and we continue to make investments in this area," said Phil Holden, the director of MSN communication services.