It is estimated that at least one-third of all junk mail messages is being relayed by home computers and to make matters worse your humble home PC was probably turned into a spam-spewing relay by one or more computer viruses. Computer viruses have come a long way since the days when they were just a nuisance put together by a teenager with too much time on their hands. Hackers for hire are crafting viruses that can search out vulnerable machines and adding them to so-called bot nets that become a pool of PCs ready react to commands sent by anyone who knows they are there. This trend towards using home PCs as spam relays was started by the Sobig virus that first appeared in January 2003. Viruses such as Sinit, Fizzer and MyDoom have continued the trend. Spammers want to use your PC to spread their unwanted wares because it has become impossible to send millions of e-mails any other way. "You cannot effectively spam without a network of proxies," said Joe Stewart, senior security researcher at Lurhq. "You are being blocked everywhere you go." Anti-spam projects, such as Spamhaus circulate lists of the net addresses used by spammers. Many mail servers check these lists when a message turns up and drop mail from known spam domains. Viruses like Sobig and MyDoom give spammers another way to despatch messages; they get your PC to do the job for them. To cover their tracks they will use only a small number of the thousands of machines they remotely control at any one time. These malicious programs use sophisticated techniques to create a bot net. Paul Wood, chief information security analyst, at MessageLabs said this type of virus had a very distinct pattern of activity. Initially, he said, infected machines were very busy sending copies of the virus they had contracted to all the addresses found on that PC. Then, on a date coded into the virus, the machine stops looking for fresh victims and instead reports its existence into one or more locations on the net. Usually at this time the virus is updated with new code that turns it into a spam relay. It then waits to hear from its controller about what it should do next. Read up on it here:  www.messagelabs.com