Viruses aimed at Microsoft up sharply

The number of new viruses and worms aimed at Microsoft's ubiquitous Windows operating system has risen 400 percent between January and June from the same year-earlier period, leading computer security company Symantec says.

Nearly 5,000 new Windows viruses and worms were documented in the first half of the year, up from about 1,000 in the year-earlier period, said Symantec, the world's largest computer security company by revenue.

The problem of computer viruses can be massive, overloading computers with messages, automatically rebooting systems and sometimes disabling them.

Symantec said virus writers and "spammers" are joining forces to exploit and profit from vulnerabilities in Microsoft's widely used software, reiterating warnings made in August by security group MessageLabs.

In its report, New York-based MessageLabs said computer viruses spread by e-mail were growing more sophisticated as virus writers and spammers increasingly collaborate in an effort to make smarter bugs and make money.

Computer security companies, like Symantec, often pick up new trends in the secretive world of virus writers and spammers by monitoring chatrooms.

"This is the first report we've done in which there is a commercial motive," said Alfred Huger, a senior director at Symantec's Security Response team.

Huger said malicious code writers were increasingly going to spammers to sell them access to the computers that they hack, or break into.

Spammers, after forking over money to the hackers for access, then flood those hacked computers with unsolicited messages, or spam, that often advertise products or get people to spend money.