SBS 2003 Rtvscan.exe excessive CPU usage

I am looking at a domain running Small Business Server 2003. All the PC’s are running fine with Symantec AntiVirus 9.0, however; the server’s CPU usage running the Rtvscan.exe is extremely high slowing down the server.

If I stop the Symantec AntiVirus service and then restart it, all works well. After 24 hours the problem re-appears.

Any ideas? Thanks