Remote Access Trojans (RAT) are malicious programs which can compromise a person's computer, and then use it to send spam mail. Antispam and antivirus company Sophos claims that 1/3 of spam on the internet is being circulated by RAT-infested PC's.
Sophos consultant, Graham Cluley, said, "There are lots of people on cable modems and broadband connections that haven't properly secured their computer. They don't know it, but their PC is being used as a relay for sending spam to thousands."
Cluley also said that attackers using a RAT program could, "steal information, read files, write files, send e-mails from that user's name--it is as though the attacker has broken into the office or home and is sitting in front of that computer." Source