Antivirus Removing Itself from Win2000

I was wondering if anyone is using Symantec Corporate Antivirus version 8.1 We are using it at work and it shows which computers it is protecting and when they were scanned. I just recently scanned by IP group and found that a punch of my machines were not detected. Then after further investigating. I found the machines no longer had Norton Antivirus installed on them. This happened on a punch of Win2000 Pro machines. It looks like of of the classrooms are still protected and the other half is not. It is not left in the add/remove program. The students and teachers do not have permission to remove anything. I have searched on the internet but am not finding any thing. Has anybody else witnessed this?