Well, what natakaasd is right..but..I want to mention something here:-

- always scan your usb drive each time you copy/share with your friends...USB drive is one of the most popular way for virus to spread

- update your antivirus! even your computer is armed with best/good antivirus, but if you don't update it..also useless

- don't download or open files that you're not knowing where the source file came from...scan it first before use, especially those files from p2p source, such as BT, emule..etc