Backdoor.Trojan

Hi,

I have downloaded a virus somehow in the past few days. It is screwing up my pc. Won't let my change ANY system settings, cant rename CD-Drive letters and can't install SP1 (Vista). Norton Internet Security 09 keeps detecting it as a Backdoor Trojan and saying it is blocking it, but it is affecting my pc. This is a serious problem as I have been cut off from my network thus I have no internet on my pc... There appears to be no cure, I had a look on internet and it is telling me it is also known as Trojan Win32 Midgare.A but there seems to be no way to get this removed.

Anyone got any ideas to help me please! I have run Norton fulls scans numerous times but it can't pick it up, because this virus doesn't formally 'exist' on my pc until it attacks, when Norton pops up saying it is blocking it, but it is still having an effect on my pc.

After my exams this week I will have to restort to reinstalling Vista, but if I can avoid it would be much appreciated.

 

Thanks