Browser incompatibilities

I have an implementation of FCKeditor 2.0 on several live systems, and occasionally I get complaints from customers who do not see the editor load at all. Usually these are coming from IE 6.0 on WindowsXP, so it is not a simple incompatibility. The only thing I have occasionally been able to isolate is Norton Internet Security, but this is not always the issue. I was wondering if anybody had similar experiences as well as any information or guidelines as to what security configurations, etc. can cause problems. We are using the PHP integration, but I don't think this would make any difference. In all testing and about 90% of user use there are no issues at all, so it has been tricky to try to identify the problems and resolve the issues for the other 10% of unhappy users.