Email "Spamming" and Email "Spoofing"
Here is a snip of a great article explaining Virus Spoofing from www.lse.ac.uk:
Email-distributed viruses that use spoofing, such the Klez or Sobig virus, take a random name from somewhere on the infected personís hard disk and mail themselves out as if they were from that randomly chosen address. Recipients of these viruses are therefore misled as to the address from which they were sent, and may end up complaining to, or alerting the wrong person. As a result, users of uninfected computers may be wrongly informed that they have, and have been distributing a virus.†