Out of interest sake, I run Windows XP 64bit on some of my pcs and laptop. MSSE doesn't support this OS, nor Windows Server 2003 which XP x64 is based on.
So that is just 1 negative point to MSSE for me.
Personally, I don't love Norton, but the Symantec Corporate packages are amazing if you can get your hands on these. Minimal resources, no problems, 1st take action then ask you whether you want to undo the action or go along with it. I have people coming back to me asking for this software again after they bought new pcs and used whatever other suggestion people gave them.
Currently using Symantec Endpoint Protection, works by fat the best and never had problems on it with several systems, OSs ect.