Spammers are increasingly hijacking home PCs to send junk mail, according to MessageLabs.
The managed email service provider claims to have proof of spammers using viruses to plant Trojan malware on PCs to provide remote access.
Once the software is installed the PC can be used to send out spam at no cost or risk to the spammer.
"We'd speculated for some time that this may be happening, but it's always been difficult to prove," said Paul Woods, chief information analyst at MessageLabs.
"This activity is hard to spot because spammers only send a few spam mails from each PC to avoid internet service providers realising what is going on.
"The number of unshielded PCs using 'always on' broadband connections has grown, and they are easy pickings for the spammers."
It is thought that spammers are turning to home users because corporate IT managers are increasingly adept at keeping their servers from being hijacked.
The US and UK governments have made combating spam a priority, and will be holding separate hearings into how to deal with the problem.
Spam now accounts for nearly half of all emails sent worldwide.
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