Norton will kill cpu usage. Mcafee is not bad at all. I get no difference in download speed while testing with mcafee on or off but with norton my download speed dropped about 40%.
You can do a 3dmark 2006 cpu test with and without Antivirus and check : http://www.futuremark.com/download/ But i m not realy convince that an antivirus can have an impact in trading software...
All I can say that by switching Norton AV off, but keeping firewall on, X_T started functioning properly, that's just my experience so far.
Here is a good site that addresses internet security issues: http://www.grc.com - they offer an online security test (Shields Up).
Sorry for the late reply. SIF = Statefull Inspection Firewall in other words the packets themselves are inspected rather than only at the moment of establishing contact. (Hope I have explained it correctly, at least this is my understanding) Maria