a bit of pedantry... Strictly speaking, a trojan is not a virus (although AV software does deal with them). To quote from the help file from my AV software:

Trojans, or Trojan horses, are small seemingly harmless programs. To cause any damage, these programs must be installed onto your system. Once a Trojan is installed, it has all the same privileges as the user of the computer and can exploits the system to do something the user did not intend. The main difference between Trojans and viruses is that Trojans cannot replicate or spread on their own.

Jon