Norton Anti Virus Question Please

Hi everyone:

Some help please with something that purplexed me today.

I got a pop up message from Norton Anti Virus at 4:30PM that they had removed/fixed a virus Downloader.trojan and it gave the file & extension. What does the virus want to do if you don't have protection please?

I am a complete safety fanatic and was surprised by this. I constantly delete the temp files/history/cookies, run Spyware every hour and Anti Virus about every 3 hours. I only visit sites I am comfortable with and never open email that I don't know. Have Micorsoft updates automatically sent.

What made the computer get the virus? What did I do wrong please and after I ran Norton and Spybot and both came up clean. Does this mean it is 100 percent gone as if it never happened?

Thank you for your time and education!!