You should *never* have multiple antivirus, real time spyware protection, or firewalls running on a PC as they *will* conflict. You may have resource problems, system instability, or unexpected behavior. If you believe the CA product is better, remove OneCare. No protection is 100%, but it is certainly possible that the CA product is "catching" more because it *is* already interfering with OneCare and is intercepting the activity before OneCare can ever scan the file.
There is no solution except to remove one of the two suites.
-steve