Actually, a trojan is a virus that comes "packaged" in a file that can be legitimate. For instance, a trojan can attach itself to a normal software program. When you run the program, both the trojan and the program are launched so it appears as if there is not a problem.


Effects and threats vary by virus. The more common ones are just annoying more than anything though some have the ability to cause serious damage to your computer ( not physically of course ).


The best thing you can do to protect yourself against trojans and other virii is to get antivirus software from an established vendor that updates regularly ( Norton comes to mind here ).