the virus is from somthing that you downloaded...especially since you said you coincidentley found the backdoor in your computer.....
But...you should check your start up...and uncheck anything that looks unfamiliar, do so that it (the virus) does start up when you start up your machine...like a virus does.....
Also....while looking at your start up you might want to go to THOSE files that look unfamilair and see wher it its and when it got there....then you can tell if it came iwth your machine..your add on....or some thing that just pooped up on April blah blah 2002............
Finally.....have someone show you how to edit your registry.....if you can do this AND the startup thing...then you can rid your computer of that Virus...once and for all!!!!!