forget,
There is a small delay between the time a Virus/Trojan/Worm is released into the wild and the time before the Antivirus Company has a patch for it. There is a further delay in the time the patch is available and when your Antivirus Program downloads/installs it.
So there is always a small window for a virus to get in. If you ever download anything, update your Antivirus Program and scan the file before opening it. That is your best chance to keep from catching something on your computer.
Also you could run an independent virus program just to double check your system from time to time. I always recommend:
Trend Micro's HouseCall
As a second line of defense for your computer.
Take Care,
Richard