The issue with Norton Anti-virus and McAfee is somewhat technical. The non-technical answer is those programs make changes to your computer that can cause corruption to your data. The technical answer is those programs make constant changes to the registry and the Borland Database Engine that will...cause corruption to your data.
There are other options out there. We recommend Avast Antivirus at this time because it is just as effective as the big guys but it doesn't use the BDE to get it's job done. The location for this software is http://www.avast.com.