The number of new viruses and worms aimed at Microsoft's ubiquitous Windows operating system has risen 400 per cent between January and June from the same year-earlier period, 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 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 email were growing more sophisticated as virus writers and spammers increasingly collaborate in an effort to make smarter bugs and make money.
Computer security companies such as Symantec often pick up new trends in the secretive world of virus writers and spammers by monitoring chat rooms.
Alfred Huger, a senior director at Symantec's Security Response team, said: "This is the first report we've done in which there is a commercial motive."
Huger said malicious code writers were increasingly going to spammers to sell them access to the computers that they hack.
Spammers, after forking over money to the hackers for access, then flood those hacked computers with unsolicited messages.
Symantec also said it expects more viruses and worms in the future to be written to attack systems that run on the Linux operating system and hand-held devices as they become more widely used.