Twice lately when ive been on holiday my email address has a sent a virus out to everyone in my address book. Each time the pc hasn't been on for days before it. How can that happen? I don't have any viruses
some viruses "spoof" sender's addresses by harvesting them from the victim's machine. If you know you're clean, then it could be that the infected machine is owned by someone who has you - and the person who received "your" virus - in common.
If your antivirus program has a "scan at bootup" option try that as the viruses don't load till after bootup.I have found a couple doing this after my pc started acting strange.