How to take out pesky Trojan Virus?

Hi,
I am using a laptop at home and my Antivirus is Norton 2009. This Antivirus needs to update through the internet. My friend inserted her flash drive for copying a file from my computer. A virus detected on my computer. I scanned the flash drive as well as the computer, but the virus is still there! It's the Trojan virus which was detected on my computer. I fully scanned the computer but still the virus is in there. How can the virus be removed from the computer? Apart from formatting the computer, what else can we do to remove the virus? Is there any other software that can fully remove the virus? Answer please. Thanks.
