one of the best things you can do for virus prevention on a windows computer is turn off that dang hide known extensions. you don't have to worry about .jpg or .gif, it's the ones where they put .jpg.scr or .gif.exe with the hidden extentions you see the .jpg but not the .scr.
To do this (i'm working from memory here, I'm on my mac) double click on "my computer" (XP users can click on start then "my computer") then go to either view or tools (depending on your version of windows, it's the former for 95 and 98 and the later for ME, 2K, XP) and click on folder options. Click on the view tab. In the advanced settings you will see a check box for "Hide file extentions for known file types". Make sure that there is no checkmark or X in this box.
For those that use Outlook Express (or any other mail reader that allows you to preview with out opening( you should turn off the preview pane. This is under the view | Layout menu in Outlook Express, I'm not sure where it is for other programs. What this does is prevents malicious html scripts from running and also can cut down on spam because you don't trigger the magic cookie by reading the preview.