The Sober worm first appeared in 2003 and can hijack a Windows-based computer and force it to repeatedly send spam e-mails. The continuous e-mailing can lead to overloaded servers and reduced network performance. Last month, a variant of the Sober worm was spread as an attachment that claimed to be an old class photo sent by a schoolmate.

A variant of Sober known as Win32/Sober.Z@mm is pummeling servers at Hotmail and MSN with "unusually high mail load," causing delays in e-mail delivery to Hotmail and MSN customers, said Brooke Richardson, MSN's lead product manager. Richardson also indicated that Internet service providers besides Comcast may be having problems directing e-mail to Hotmail and MSN servers.

Source:-  http://news.com.com/