Virus confussion??

I am having an issue with my Norton Internet Security 2010 that support could not rectify.

 

I am running Widows XP sp3. I have Windows updates as well as Norton updates set to run automatically. I have had the Software installed and running for a couple of months now. I have performed many Full system scans during this time. Using the default settings (basically). I have a separate hard drive with approx. 120 gigs of files stored. ie: pdf's, doc's, zip's, rar's, exe'setc.

 

These files have been scanned at least twice. Once as I downloaded them (I have Norton set to scan downloads before they are saved) and again during my scheduled weekly system scans. Up until today they were all passed ny Norton as not containg any viruses. After scanning today, I am getting alerts of viruses from files that have been scanned and passed numerous times.

 

Any ideas as to what's up that and,or any solutions for prevention would be appreciated.

 

Thanks, wrecker69