there are trojan visuses and trojan RAT's, a RAT is a Remote Administration Tool that piggybacks onto your computer from installing (mostly) pirated software, they are free but come at a price. Some have been known to come from shareware. Once installed on your computer and if you have broadband get's pinged from various online souces to see if you have a RAT installed, if you do and you don't have a firewall, under the right situations you computer becomes a tool for a hacker. Your computer can then be used for Denial of Service attacks or the Remote User can do whatever even view what's on your screen. The best way to prevent all this is to get a firewall and just block the port that pings your computer or Moosoft like rocmonter said to remove the RAT.