I also had the same problem and was told that the virus was some sort of system designed to record keystrokes - that way whoever developed this can record your passwords, etc. I too downloaded AVG which worked far better than any other anti-virus I tried. At first (it started about 2 months ago) AVG was detecting 4 or 5 trojan files every hour or so and gradually has cleared my computer of them and I only get them once every week or two. Computer is much faster now! Definitley download the free version, and if you're in an office environment I believe there is a version for that which is fairly cheap.
hope this helps!