I run Norton 360. I can play any Flight Simulator with full graphics and never stutter. I have no problems running any CAD programs no matter how complicated. I have not had any infections or Virus's for years. I occasionally run AdvancedSystemCare and it comes up with stuff occasionally but misses a lot of stuff Norton catches. ( do a scan with one or another first and you will see what I mean). I never used MvAfee because of the problems I have seen people have with it over the years.
When I load XPSP3 my computer basically died. It was Symantec who helped me get it running and told me that the problem was with an Intel.dll entry in the registry that needed to be removed.
After contacting Microsoft, and letting them remotely take over my computer to see what was wrong they advised me not to install SP3. A day later they called me back with the same info Symantec gave me and said I could now load it. I never did. My computer runs fine and I will be switching to W7 as it ran great for the time I ran it up to the final release version. Symantec also helped me install Norton 360 on W7 even though it was a beta program. I did not have to pay a dime for their services, or for the install on W7, (Norton 360 is allowed on 3 computers). That was my experience. I have heard many others who have not had the same results. I believe them too. I can offer no explanation. Spotty service is not a good thing.