Internet Explorer

I have recently aquired a trojan horse, I believe it may be the same type as Trojan.Win32.Agent.akk. When starting the computer everything seems fine but then I open IE and it starts and then says that it has encountered a problem and closes, I have also noticed that if I move open windows around the screen it is leaving traces for a few seconds. Normally I would put this down to lots of processes or maybe lack of RAM, but my PC is brand new with little in the way of applications and it has 4gb of ram with a quad core (6600) processor. I have tried runnung AVG and spy sweeper and it is still happening, I really dont want to have to format the drive as I have a lot of media files that would need backing up and this is not a quick process with an external network drive, please could somebody help. Thanks. p.s. im running Vista Ultimate if this makes any difference.